What could have caused this?
A 2.4GHz computer with 256MB RAM (Windows 2000 Pro) was running very slowly, even after a reboot.
I opened Disk Defragmenter and clicked on Analyze.
Instead of a short wait followed by the fragmentation summary, the computer told me that it could not complete the operation and the mouse froze. The keyboard still worked so I highlighted Retry and hit Enter. The keyboard froze. Everything locked. I finally had to hit the reset button. THE HARD DRIVE WAS GONE. It was no longer recognized in the BIOS. I powered down and restarted. 'Still no hard drive recognized. I unplugged the computer and plugged it back in, and the hard drive was back. ??? All connections are solid. The computer is working, though still not as fast as it should be, and now I’m afraid to even open the defragmenter. Again, ???
Most likely the power supply is starting to go. It sounds like it is no longer able to supply a consistent, clean supply of power across the board. For whatever reasons hard drives seem to be most sensitive to this. Turning off the PSU or unplugging it altogether will tend to set things back to right by allowing the capacitors to completely discharge and recharge once power is plugged back in. I expect you haven’t seen the last of the problem, though.
FWIW I’ve had the same problems twice in the past, and both times the (cheap imported) PSU was the culprit. Buying a good brand-name PSU (imported or otherwise – Antec, Thermaltake, Coolermaster, etc.) has taken care of the problem.