Just a quick note on the "build your own thing". (from what I have seen) If you know enough to hold a conversation around here, can read instructions, and can install your own hardware (video cards, modems, NICs) you should have no problem building your own computer. You end up with a better system for less money with exactly what you want. Check out
These aren't the best prices on the net, but they have a great shopping cart on the site that will allow you to play with your configuration and give you an idea of what it would cost "out the door". (They are a local company for me and VERY trustworth)
As far as WinXP is concerned.. You should be able to pick it up for about $100 at any computer store. It's pretty easy to install as long as you take 2 important steps.
1. On the CD (and the microsoft website) there is an XP compatibility test program. RUN THIS. It will detect all of the hardware and software on the system and tell you exactly what you may have problems with and what you will need to make it work properly.
2. Don't do an upgrade. Copy off all of your critical data either on a zip drive or a CDR, format the drive and make a clean install. As with all of Windows systems, an upgrade is just asking for problems.