MATLAB, 4 bytes

ispc

From the documentation:

tf = ispc returns logical 1 (true) if the version of MATLAB® software is for the Microsoft® Windows® platform. Otherwise, it returns logical 0 (false).

There are also the functions ismac and isunix. I'll leave it to the reader to figure out what those functions do. Mego kindly asked for diagrams explaining ismac and isunix so I've tried to illustrate it here:

It was not asked for a diagram of ispc but I can reveal that the behaviour is pretty similar, except substitute OSX and Unix with Windows.

Python 2.7, 24 bytes

import os
0/('['>os.sep)

Thanks to FlipTack for 3 bytes

This program takes advantage of the fact that Windows is the only OS to use \ as a path separator. Normally this is frustrating and bad, but for once it is actually an advantage. On Windows, '['>os.sep is false, and thus 0/0 is computed, causing a ZeroDivisionError and exiting with a non-zero exit code. On non-Windows platforms, '['>os.sep is true, making the expression 0/1, which does nothing, and the program exits with exit code 0.

PHP, 22 bytes

`<?=PATH_SEPARATOR>":";`  

prints 1 if the path separator is semicolon (colon or empty for all other OSs except for DOS and OS/2), else nothing.

also 22 bytes, but not that safe:

<?=strpos(__FILE__,92);

prints a positive integer if the file path contains a backslash; else nothing.
A safe alternative with 27 bytes: <?=DIRECTORY_SEPARATOR>"/"; prints 1 or nothing.

Strange: <?=__FILE__[1]==":"; (20 bytes) should be, not safe either, but ok. But although __FILE__ pretends to be a string (I tried var_dump and gettype), indexing it throws an error (at least in online testers).
I´ll try it on my server tomorrow. Will someone test it on Windows?

27 bytes: <?=stripos(PHP_OS,win)===0;
tests if predefined PHP_OS constant starts with win (case insensitive; Windows,WIN32,WINNT, but not CYGWIN or Darwin); prints 1 for Windows, else nothing.

17/18 bytes:

<?=strlen("
")-1;

prints 1 if it was stored with Windows linebreak (also on DOS, OS/2 and Atari TOS - although I doubt that anyone ever compiled PHP for TOS), else 0.

You could also check the constant PHP_EOL.

more options:

PHP_SHLIB_SUFFIX is dll on Windows, but not necessarily only there.
php_uname() returns info on the operating system and more; starts with Windows for Windows.
$_SERVER['HTTP_USER_AGENT'] will contain Windows when called in a browser on Windows.
<?=defined(PHP_WINDOWS_VERSION_BUILD); (38 bytes) works in PHP>=5.3

conclusion

The only failsafe way to tell if it´s really Windows, not anything looking like it, seems to be a check on the OS name. For PHP: php_os() may be disabled for security reasons; but PHP_OS will probably always contain the desired info.

C, 44 43 38 36 bytes

Thanks to @Downgoat for a byte! ^{crossed out 44 is still regular 44}
Thanks to @Neil for two bytes!

f(){return
#ifdef WIN32
!
#endif
0;}

Mathematica, 28 bytes

$OperatingSystem=="Windows"&

Java 8, 33 bytes

Special thanks to Olivier Grégoire for suggesting separatorChar, and Kritixi Lithos for -1 byte!

This is a lambda expression which returns a boolean. This can be assigned to Supplier<Boolean> f = ...; and called with f.get().

()->java.io.File.separatorChar>90

Try it online! - the server isn't windows, so this prints false. However, in my windows machine, the same code prints true.

What this code does is get the System's file seperator, and check whether its codepoint is larger than the character [. This true for Windows, as it uses \ as the seperator - but every other OS uses /, which has a lower code in the ASCII table.

J, 7 bytes

6=9!:12

This is a verb (similar to a function) that uses the builtin foreign conjunction 9!:12 to acquire the system type where 5 is Unix and 6 is Windows32.

Befunge-98, 7 bytes

6y2%!.@

Try it online!

This works by querying the system path separator, which is \ on Windows and / on other operating systems.

6y            System information query: #6 returns the path separator.
  2%          Test the low bit - this will be 1 for '/' and 0 for '\'.
    !         Not the value, so it becomes 0 for '/' and 1 for '\'.   
     .@       Output the result and exit.

Perl, 15 bytes

print$^O=~/Win/

Outputs 1 on windows, nothing on another OS.

Note that I'm not using say as it adds a trailing newline, which is truthy in Perl.

Batch, 50 bytes

@if %OS%==Windows_NT if not exist Z:\bin\sh echo 1

Edit: Fixed to ignore DOS instead of claiming that it's Windows.

The only other way I know of running Batch outside of Windows is to use WINE which by default will map Z: to /. Therefore if Z:\bin\sh exists, chances are that it's /bin/sh, so not MS Windows.

I don't know what WINE sets %OS% to, but if it's not Windows_NT then I could save 23 bytes.

Node.js, 27 bytes

_=>require('path').sep!='/'

Perl 6,  19  18 bytes

put $*DISTRO.is-win

put ?($*CWD~~/\\/)

Both output True␤ or False␤ depending on the system it is run on.

julia, 10 bytes

is_windows

A function that returns true for windows

JavaScript, 42 30 bytes

console.log((
//Begin
_=>/Win/.test(navigator.oscpu)
//End
)())

Edit: saved 12 bytes thanks to Neil.

Excel VBA, 41 Bytes

Immediate windows function that returns if the true if the name of the operating system starts with W. This will only ever be true on Windows based systems as Excel VBA is restricted to Windows and OSX.

?mid(Application.OperatingSystem,1,1)="W"

R, 21 byte

el(.Platform)!="unix"

Haskell, 32 bytes

import System.Info
f=os=="mingw"

puts [string match windows $tcl_platform(platform)]

puts [string match W* $tcl_platform(os)]

assuming Windows is the only system the name begins on a W.

3rd attempt:

tcl, 26

puts [info exists env(OS)]

assuming Windows is the only system the OS environment variable is defined.

tcl, 38 bytes

 expr [lsearch $tcl_platform windows]>0

PHP 17 Bytes

The following will output 1 if windows and nothing if anything else. Ignoring notices of string convertion.

<?=PHP_OS==WINNT;

Try online Online tests for linux because the sandbox is linux for PoC.

FPC, 65 chars

begin{$ifdef win32}writeln('f');{$else}writeln('nf');{$endif}end;

8th, 11 characters

 
os 1- not .
 

Ungolfed version (with comments)

 
G:os  \ Return a number n indicating the operating system 
      \ 0 for Linux
      \ 1 for Windows 
      \ 2 for macOS
      \ 3 for Android 
      \ 4 for iOS 
      \ 5 for Raspberry Pi
n:1-  \ Subtract 1
G:not \ If Windows --> true, otherwise --> false
.     \ Print result
 

Dyalog APL, 21 bytes

'W'=⊃⊃⎕WG'APLVersion'

⎕WG'APLVersion' system function Windows Get property APL Version

⊃ pick the first element (Target Environment)

⊃ pick the first letter (Windows/Linux/AIX/Solaris)

'W'= W equal to that letter?

Java 8, 49 bytes

()->System.getenv().get("OS").contains("Windows")

Longer than the other Java answer, but takes a different approach.

This lambda fits in a Supplier<Boolean> and can be tested with the following program:

public class DetectMSWindows {

  public static void main(String[] args) {
    System.out.println(f(() -> System.getenv().get("OS").contains("Windows")));
  }

  private static boolean f(java.util.function.Supplier<Boolean> s) {
    return s.get();
  }

}

Vim, 2 bytes

<C-a>1

On Windows, <C-a> (ctrl+a) is mapped by default to Select All. If you type a 1 in select mode in Windows, it replaces the selection with what you typed (1) leaving a 1 in the buffer.

On other operating systems, <C-a> by default is mapped to Increment number. Because there's no number to increment, it's a no-op, and then the 1 increases the count but in terms of the buffer is a no-op.

1 is truthy in Vim, and an empty string is falsy

FPC, 71 chars

begin
{$ifdef win32}
writeln('f');
{$else}
writeln('nf');
{$endif}
end;