I don't know what he ( your roommate ) put in it, or how. Did he just reinstall your system, or did he put a completely different system in it. If he reinstalled it, that means he did a system recovery, which would put the factory system back on clean.
Try doing a system recovery of your own. At start up, keep pushing the F10 key until it reaches the screen for system recovery. Start pushing the button as soon as the power supply fires up, & the fan & hard drive start spinning.
Next, you want to select the "Advanced" tab to get to the "Destructive" option. Choose this & a screen will come up warning you that all files will be lost/ removed, something like that. This is the option you want. You'll be removing everything except your "D" drive, where the recovery section is. A complete Windows reinstall.
Follow the Ques as they come after a few minutes, if there is still a "D" section on your computer. ( Again, I don't know what your roommate did! This is what he SHOULD have done! ) It's quite simple, & for the most part, you just keep hitting enter, except for your registration name, & workgroup. Windows will restart itself, load & go through the installment process part 2.
Eventually, it will restart & want to take you through that mundane tour, but you can click out of that.
After that, go to control panel> ad or remove & remove any unwanted programs. ( like XP tour, AOL, outdated anti viruses, ect.. )
This is all providing your "D" drive is still there, I hope it is. It will give you your original LEGAL Windows XP back!
Good Luck, I hope this helps! :-)