...just noticed your query, tissy.
I looked at Amazon, and reading the reviews it seems that it is possible to use the OEM version of xp, if a bit of a pain to register it. It may be worth this hassle though, as it is a LOT cheaper.
...I can vouch that Windows works on my iMac (I installed vista on it as an alternative, just really to see if I could. Now THAT is geeky). It is very easy, as if you search for "boot camp" using the spotlight feature it gives you a printable and easy to follow installation guide. It will use a fair bit of hard drive space though, I think 12 gb is the very minimum, I partitioned 32 gb (out of 250) on my iMac...
....though being this geeky I feel I should still be living with my Mum and have an extensive collection of superhero comics.