A fragmented disk rarely has a significant impact on performance, but you should rule out the possibility first by running Windows' Disk Defragmenter. If your XP system has less than 256MB of RAM, a likelier cause is that Windows is using your hard disk as virtual memory too often. Install more memory, and the problem will probably go away.

Mysterious disk activity may be due to spyware running on your PC. Programs like Ad-aware and Spybot Search & Destroy can help remove those nasty little freeloading apps.

No infections? Excess disk activity and abnormal noise could indicate a defective disk at work. Back up your files immediately and replace the drive.