WinXP Pro may not be necessary for you...I'd suggest WinXP Home Edition. A little cheaper. Second the upgrading the computer advice. WinXP does require some good horsepower to run.
At a minimum for photography work (viewing, editing, etc.) consider the following specs:
1. 1.5 GHz processor (AMD or Intel is fine)
2. 512 Megabytes of RAM memory
3. 100 Gigabytes of hard disk storage capacity
4. DVD/CD burner for making backups of your photos, etc.
5. A good monitor (video screen)
6. A good mouse
7. A good keyboard
8. High speed interenet access, so you can surf these boards quickly!
9. Compact Flash Card Reader
You can put together an entire setup or buy a good one from Dell or Gateway for under $700.