Welcome to the boards, I would second what Calum has said.
Also be aware that UT2003 is available for Linux (in fact it is probably on your CD right now, if you have UT2003, and you did not even know it). Of course you could always game on a console.
If you want to buy new from scratch there is also Mac, which is very good and runs most of the games you would want, but I am an Open Source advocate, I believe that provision of source code is my right as a consumer, on every product I use. The same way ingredients lists are available on food.
If you are running a M$ windows PC then your options are Linux (one of around 250 versions), OS/2 (costs a lot and not as good as Linux - opinion), BSD (three main versions, a varient of UNIX, condidered more for use on servers and requires a little more technical skill) and a few other types of UNIX and some small development OSs. As a noob I would suggest a main distribution of Linux such as Mandrake. You can download it for free and try it out, if you like what you see you could buy a full version with some manuals and support for less than a M$ OS. If you know someone who can help you out then so much the better. You do not need to be programming guru, in fact many users of Linux are not, they use their computers same way you probably do, e-mail, internet, games.
The reason you may see many people who are Linux and programmers is because the Linux environment actually gets you interested in computers, and people often keep on learning because it is more inspirational than any other OS I have ever used.