How to uninstall windows?

Ok, if I had a way to concisely type this into a search field on google I wouldn't bother you guys on here with this question but I'm not sure how to do that. This is my story:

My room mate's computer was really screwed up and we couldn't even reformat it due to various reasons. Our best idea was to take his hard drive out of his computer and reformat it and install Windows 7 on it while in my computer. This worked, the computer runs a billion times better than before now that we returned his hard drive to his computer. The only problem is that now MY computer thinks there are two different Windows 7 installations present-- The one already there and the one "added" when I installed it on his hard drive. The strange thing is...that hard drive doesn't even exist in my computer anymore. The annoying thing is that the bogus Windows 7 install is the one in the default position in the boot menu of my computer so if I just let it automatically choose the first option it'll say "couldn't find ****" because...well because there aren't any files for it to find because the entire freakin hard drive is gone. So....I would have assumed that the only place the computer would know to look for the presence of an OS would be on each respective hard drive, I have no idea where the data is that I need to zap in order to stop my boot up menu from displaying a real Windows 7 and a bogus Windows 7.