Well, your probably going to need some sort of IR port for your PC, most laptops have these but normally desktop PC's dont for some surious reason.
Secondly you then probably going to need some software which will recognise the incoming signals and allow you to map them to certain commands in your PC. I am personally not aware of any.
FInally you will need a remote control, and one that can learn functions or be programmed.
The best one i know of for the Price is the One4All Kameleon which come in at around 60 pounds (UK) if bought on the net, othere than that the Philips Pronto which is probably the mutts nuts but would set you back around 150 notes.
Neiter of these seem worth it though just to control you PC.
Creative have a Control Panel for their PLatinum cards which sits in front of the PC and has a remote control port - but i cant imagine its cheap either.