I also use MSE on my Windows 7 desktop, because it came with it and I read that it is very good (and free) but I can highly recommend AVG free version. I use it on my laptop which has Windows XP, just because it is old and slow anyway and I was not sure MSE would work with it. AVG will try to get you to pay for a "better" version, but the free one works great, has auto updates, and has kept my laptop clean as a whistle.