Thats a hard one to do... unless you have a SPDIF connector on your computer. This would be an orange RCA style digital audio output. You could run that line out to a Digital audio input on your Receiver (which is what I am thinking you have. A device that you can switch inputs such as DVD, Bluray, Radio, Ipod, CableTV etc...)
Depending on the outputs from your computer, you can get a 3.5mm to RCA stereo converter cord, then you can use the 3.5mm (green) audio out port and convert it to the standard Red and White RCA stereo plugs and run that to one of you inputs on your receiver/amplifier.
If you have 5.1 surround out of the PC using only 3.5mm jacks then its going to be a little more difficult. But lets see what you are working with first!