www.monoprice.com
they have about the cheapest prices you will find for a kvm switch . they have a 4 port manual for 28$ shipped , if i remember the price right . and that it comes WITH the cables . it is manual so you will have to switch between the pc's yourself with the pushbuttons . and it will only do up to 1024x 768 . so if you have a small 15" monitor it will work great .
they also havd a 4 port auto kvm switch WITH the cables for about 47$ shipped . thats is a really good deal also . most places want that much for the switch alone . and you get the cables with it for that price .
i have bought from them 3 times sofar and had NO problems with them . i got my orders within days of paying for them so i would recomend them .
there are also programs to do it also but they worry me because it gives hackers a way in . some people luv them and haven't had any problems with them . but me personally , i don't like to use them . i think one of the programs is called tight vrc or something like that . look around in the FaD forums i know i have seen it talked about in there .