While you can't buy a Windows XP PC anymore, the 9-year old operating system is far from dead. Microsoft still supports it, as far as keeping the Windows Update site running. Also, on non-hyperthreaded/single CPU systems, it is probably the fastest OS. However, that could still be quite slow, if your PC is exactly as it was when you bought it (256mb ram, 20gb hard drive). When you first brought that computer home, it probably would've been quite snappy, but 9 years of Windows Updates and memory-hungry browsers have also bogged down the OS. With all the latest updates, you'll need more hardware.
I had enough parts to rebuild a PC for a friend, and put XP Pro on it. The original hardware was an Athlon 1600+ (1.4ghz), 512mb ram, 20gb 5200 rpm hard drive. It worked, but after any kind of extended use, it started madly thrashing to disk. I decided to put in a few upgrades to see what kind of impact it would have.
Startup time (from Windows logo until the red shield appeared to indicate a lack of antivirus): 1:03 minute
IE startup: 22 seconds
Chrome startup: 42 seconds
Power off: 43 seconds
Let's bump that up to 1gb of ram to reduce the thrashing:
Startup time: 47 seconds
IE startup: 37 seconds
Chrome: 31 seconds
Power off: 19 seconds
Ok, that's better, but let's substitute for a 7200rpm 40gb drive:
Startup: 36 s
IE: 33 s
chrome: 8.8 s
Power Off: 12 s
So, if you (or possibly your parents) are running XP on anything less than 512 mb of ram on a 5400 rpm drive, you could more than double the performance of the computer just by throwing more ram and upgrading the hard drive. Of course, sourcing the parts may be a problem (stores stopped carrying IDE drives about 4 years ago), but if you have another dead PC somewhere, you may be able to combine the parts to make something usuable, for free.