jQuery. How can i rewrite this code

Use the conditional operator and combine all the .css statements into a single map:

$x.css({
    'borderTopWidth':    $('#borderT').is(':checked') ? x : 0,
    'borderRightWidth':  $('#borderR').is(':checked') ? x : 0,
    'borderBottomWidth': $('#borderB').is(':checked') ? x : 0,
    'borderLeftWidth':   $('#borderL').is(':checked') ? x : 0
});

You could even move that conditional check to an external function, although that won't make the code any more efficient, just easier to read:

$x.css({
    'borderTopWidth':   getBorder('#borderT'),
    'borderRightWidth': getBorder('#borderR'),
    'borderBottomWidth':getBorder('#borderB'),
    'borderLeftWidth':  getBorder('#borderL')
});

function getBorder(id) {
    x = "5px";
    return $(id).is(':checked') ? x : 0;
};

http://jsfiddle.net/mblase75/hdpZn/1/

You can use an object as a map and using it like an array with the key as the id and value being the css property.

var map = { 
    borderT: "borderTopWidth", 
    borderR: "borderRightWidth",
    borderB: "borderBottomWidth",
    borderL: "borderLeftWidth"
}

function(id){
    if($('#' + id).is(':checked'))
    {
        $x.css(map[id],x)
    }
    else{
        $x.css(map[id],'0')
    }
}