If you're going to buy a prebuilt, you basically have to go with windows 8 now, there are probably a few that still use windows 7, but most, if not all, companies have stopped making computers with 7. That is, assuming you want a computer with current hardware'n stuff.
Edit: also, building it yourself is a cool learning experience, I know way more about how a computer works now than before I started looking into building my own (not implying I know a ton about how computers work).