I have an older machine running XP Pro and it's running extremely slow even after mulitple reinstalls. I constantly get blue screens, especially when downloading and right before it's finished doing a defrag. I've run tests on everything else and it seems to me like this is the harddrive failing. I really hope it is just the drive, and it would be a nice easy fix.

Sounds like it might be overheating.Have you cleaned out the heat sinks and fans inside tha case?.If its not that it coud be the harddrive.Try windows check disk

http://support.microsoft.com/kb/315265

I've run check disk, and it shows 4kb in bad sectors, but those bad sectors have been for a long time, this problem is relatively new. I wish I had an old harddrive to throw in the system to test it out.

Did you do what he said and try checking for overheating? My old P4 server got BSoD's out the wazoo because of overheating. Cleaning off the CPU and heatsink, then applying some fresh thermal paste did the trick.

I would check to make sure that all fans are running. Especically the CPU fan.