i'm guessing you've bought a PC that has video input sockets so you can plug a console in to the PC and run the console on the PC's monitor correct?
what type of cable are you trying to use and in to what socket? (names please, ie component, VGA, s-video, composite etc.....)
if you just mean you want to stream video and audio from the PC TO the 360 and output via a normal TV, then you need to use a cat5 network cable between the 360 and the PC, or from the 360 to a broadband router that the PC is also plugged in to......you dont use a video cable for this.....