I switched from windows xp to 7 as well. I had tried vista on my gaming machine, and it was too slow to play tf2 to my standards, so I removed it. Windows 7 manages to still hit over 120fps with maxed out settings on a 1920x1080 display, so it still has great gameplay even if it is a tiny bit slower (I saw about a 20-40fps drop when I switched, but I didn't max out anti-aliasing before so that might account for it) it still is fast enough to easily handle the game. As well, the only time I got it to have problems is when I accidentally put it to sleep while overclocking it and something caused an error that forced me to take a while to get it to wake up again (I would strongly reccommend removing the sleep setting from the computer, Microsoft hasn't been able to fix it since they made it and playing russian roulette with your computer every time you use it isn't fun).
I am currently running the full release of Windows 7 professional, but all the features that I really use on the gaming machine are available in the home version. If you get it OEM, you won't have to pay too much and can put it on one machine (Or do the old stand in of calling it in and saying it is only on one). If you are a student, you can join Microsft's MSDN academic alliance and get it for the about 20 dollar subscription fee, although I do not know how exactly to do that individually.