I have been trying to wean myself off my M$ dependency for some time. I did try lots of versions of Knoppix, which isn't bad, but frustrated me. I am really pleased with Ubuntu which is very easy to install and even easier to run. But don't just take my word for it - watch this video
This system is a lot cheaper than Vista - either as a new install or upgrade. It's free.
Couple of things to know before you start. If you must have the latest and greatest hardware and peripherals, then it may take a while before the Linux community gets around to building drivers. But if you are like the rest of us and trying to make your existing kit last a bit longer, you should have no problems. Linux runs on FAT32. While it can read ntfs, writing may cause issues. There is software that enables this on Knoppix but not as far as I can determine so far a way to do this yet with Ubuntu. Maybe someone reading this will bring me up to speed if I am wrong.
Linux boots and shuts down much faster than Windows, is much safer (very few malware writers have got around to trying to bring it down or to crack it and as it is community supported and open source is a lot more secure) and takes up much less disc space. If you have got fed up with Windows nagging you about deleting programs from your hard drive just so it can make thousands of back ups for itself, then Linux is for you.