You CAN use a PC to do this but it is pain to configure. You best bet is to buy a AP or any cheap router and use it as a AP. There is a sticky on this forum how to use router as AP. You would plug the AP into the end of the cable in each room and they can either connect to the AP via wireless or connect via a cable. You would want to put the AP on different radio channels to avoid interference. You can use the same of different SSID, same is easier but you have more control if they are different.
Cost wise it will be a little more to use a router than buy a USB card for the PC but I suspect the cost of leaving a PC powered on compared to a router will wash that costs out quickly....besides all the headaches to troubleshoot using a PC as AP.