You can buy any mic for your PC but if you buy an expensive vocal mic (from a music shop) you will need an adapter to convert it from a mono jack (or XLR) to a mono mini-jack. I would go for the cheapest you can fine (I bought a desktop one for �2.50) and plug it directly into the mono mic-in mini-jack socket of your PC sound card. Also, I have heard that the headphone/mic combinations are pretty good. Check PC-World.
It all depends what your intending to use it for....if it's only for web cam use or for Viavoice or other speech recognision packages then a headset mic is the best bet but if you have any home recording aspirations then you'll be far better spending a bit of money on a good mic. Try these on for size....the industry standard is the Shure SM58 approx �80-�90 (which i personally use)....the AKG D190...audio technica and sennheiser are aslo very good makes and anything above say �50 should give you very good quality. For these mics though you'll need a XLR male to 3.5 ml jack lead to connect to the soundcard on your PC.