Just be carefull about some of the companies, their are some bad ones out there. Seems like Newegg is about the best combination of good business practicies and cheap prices (one in the same in a sane world). But that's not to say their are equally respectable companies out there, some with probably some good deals going on.
Caveat Emptor, baby
Also the hundred bucks is a pretty resonable price for a OS, of course I don't pay for OS'es (well I do.. but only when I am impressed or in a good mood) since there are plenty of people out there willing to give OS's away for free. (and I am not talking about those warez bastards either, or p2p networks, can you guess my secret? I'll give you a 2 hints, it involves flightless birds in some way and little guys with pitchforks from the northwest)
Anyways if you like using XP you should also like giving money to Microsoft, after all it's them that went thru all the trouble of making it in the first place. It just doesn't make sense to me why so many people figure they can't do without Windows, but then turn around and try to rip them off at every opertunity.
Don't worry I am not trying to slight you in any way for trying to find a bargin, far from it. It's a sign of intellegence. (although trying to use your freinds copy was a bit shady) It's just a idiot (me) pointing out the obvious again. Forget I said anything...