VBS Scripting Error

I've reformatted the code for easier readability. The single apostrophe ' changes everything behind it into a comment, so it's not part of the code. So '"pst"'" isn't visible.

Actually, there are more problems than just that. That whole line is formatted incorrectly, and I think you've got a couple other lines out of order. It should look like this, I think:

strComputer = "."
Set objWMIService = GetObject("winmgmts:\\" & strComputer & "\root\cimv2")
strsql = "Select * from CIM_DataFile Where Extension = 'pst'"
Set colFiles = objWMIService.ExecQuery(strsql)
Set fso = CreateObject("Scripting.FileSystemObject")
set wfile = fso.opentextfile("c:\test.csv",2,true)

For Each objFile in colFiles
    Wfile.writeline(strComputer & " " & objFile.Drive & " " & objFile.Path & " " & objFile.FileName & "." & objFile.Extension & " " & objFile.FileSize)
Next

You need param list (), if you call a function to receive its return value; and _ continues a line - so change:

Set colFiles = objWMIService.ExecQuery _
strsql = "Select" * from CIM_DataFile Where Extension = '"pst"'"

to

strsql = "Select * from CIM_DataFile Where Extension = 'pst'"
Set colFiles = objWMIService.ExecQuery(strsql)

or:

Set colFiles = objWMIService.ExecQuery( _
     "Select * from CIM_DataFile Where Extension = 'pst'")

After reading @Joe's (+1) answer, I tried to clean up the quoting in your SQL.