I'd like to endorse the comments about linux, especially by citan. The time has now certainly come for anyone with an experimenting turn of mind to give a linux desktop distribution a try. There are hundreds of them now, all with with their different strengths, all of them legally free. If you want to make a hobby out of it, you can try as many as you want.
A good starting point is
www.desktoplinux.com. Lots of info, but a bit ridden with techie jargon.
Raycary, you can buy your PC's with windows, but you'll probably get vista whether you want it or not. It's a turkey and you may soon get fed up with it. It's easy to install a linux system later on, in a dual boot configuration. My choice is Ubuntu, which I now use almost all of the time. You could download it as an iso file (but very large!), and make a live cd from that. Boot up from the live cd and give it a try! (Of course, it'll be slow like that.) If you like it you can install it in it's own partition (30 gig would be more than enough to start with), then boot as you want from the 'grub' menu.
Strengths of Ubuntu: Nice clean desktop, easy to use. In windows you're forever scanning for plants etc, you never stop. None of that with Ubuntu, just use it, it works. Large no of applications with the live cd, incl openoffice; you don't need to shop around for all that stuff separately. Large repositories of downloadable packages. Massive amount of online help, especially the community docs.
Downsides: Not guaranteed to work well with all peripherals, although it's improving. (Try out your printer, etc, with the live cd.) A bit fiddly to set up for multimedia, lots of codecs to be downloaded, but there are applications (eg Automatix) that make it easy for you. Not yet up to speed for high-end windows gamers.
The scary talk about the command line is rather exaggerated, in my view. You can easily learn the few command strings that you do need.
So there you are, start with windows, then you can experiment with linux distro's at you leisure. Enjoy!
The age of windows is over. The age of linux has come.