While most people have covered most of the bases here, I will say one more thing I didn't see mentioned. Linux (At least most distros of it) is much more efficient than Any of the Windows OSs I've dealt with. The main reason I got into linux was because I had an old laptop that wasn't getting good performance from Windows XP, even on a fresh install. My old Toshiba Tecra (6100 I think) wouldn't even play video without stutering in Windows. I gave linux a try, and I probably made a huge mistake starting out with slackware, but once I got everything working like I wanted it too (Took several months of trial and error, but I like that kind of computer learning.) it ran most video (up to a certain resolution) fine. I also noticed the simple stuff like word processing programs saw an improvement as well. I'd also managed to get a SNES emulator to work in linux when in windows it was practically unplayable. (zsnes in both cases. I could however, reboot windows into dos mode and get decent performance with dos zsnes, but who wants to do that every time?)
More recently I purchased an early generation netbook. The Eeepc 900. It was only slightly more powerful than the old toshiba tecra, so Linux was the go to choice there, as well. I tried the distro that came with it but found that the "Easy Mode" was too watered down and the "Full Mode" wasn't what I wanted, so I put a netbook centric version of Ubuntu on there and it's been working fine. It's probably more bloated than I need, and I have considered trying something else, or even going back to slackware, but haven't found a good reason to go through the trouble. It does everything I expect it to.