why is it that when I went to CompUSA, they had one single Bluetooth adapter?

The problem is that on the PC side it still seems to be the chicken and the egg problem. Apple users have enough reasons to actually use bluetooth, so it is becoming commonplace. Lack of real uses on the PC side combined with stories like this don't help. I remember USB being the same way. It took forever to take hold on the PC side, somewhat due to a buggy USB implementation in Windows 95, and not enough devices for people to justify it.

Oh, and the worst part is we decided to give in to Microsoft's manipulations and installed XP on my girlfriend's machine. I read somewhere that they now allow you to use one copy of XP on up to something like 3 machines. Is that true?

Not that I know of. All boxed copies of Windows XP are single license installs. Office XP on the other hand can be installed on 1 desktop and 1 laptop that a single owner uses. I wonder if Microsoft will ever release a Family Pack of their OS.