Later this year, there will be two new operating systems available.
Microsoft are going to release Windows 7, and it looks good. (I have downloaded the release candidate and installed it on a computer. While I need to test it a bit more, I do think I will be happier with it than I am with Vista.)
Apple also have the next iteration of OS X almost ready for release. I havent used a pre-release version of "snow leopard", but the reviews look good.
Of course, OS X only works on Apple computers, but it also will cost a lot less.