I would look at ebay for most of the hardware components. A little over a year ago, I managed to get an ATI Radeon 9200SE with 128MB DDR RAM AGP graphics card (new) for $40 and a Soundblaster Audigy2 card (the one with a firewire port) for $50 (used, but with everything originally included in the box).
Also, when selecting your components, go for quality over price (avoid off-brand parts). It's better to spend a little more money on a part made by a trusted company than to buy the off-brand equivalent (that probably doesn't work quite right) and have to replace it shortly after the warranty runs out (if it even has a warranty). After having to replace my motherboard over two years ago due to leaky capacitors, I will only buy ASUS motherboards now.
If sound quality is important to you, buy a sound card and try not to buy a motherboard with on-board sound. You can't go wrong with Soundblaster. Soundblaster Live cards are very inexpensive and would probably be perfect for your system. You can spend a little more and get an Audigy or Audigy2 card, but Live would probably work just fine. Also, Turtle Beach makes good quality sound cards. I would recommend checking out their cards as well.
By the way, XP doesn't always come preloaded with the drivers you need, expecially for brand-name computers. As I said earlier, HP computers have been particularly troublesome. I have never been able to install Windows XP from an ordinary installation disc onto a HP computer made in the last few years and have full sound and video capabilities because it doesn't have the appropriate drivers, and you can just forget about downloading them from the internet. HP's website is horrible and it's a waste of time to look for drivers there. I'm not sure, but the best that I can figure out is that if you need drivers, HP wants you to pay them to mail you a CD with the drivers that you need
. HP's calculators, printers, and scanners are OK, but avoid all other HP products like the plague.