I am posting this because this saved my bacon 2 years ago, and again today, and I don't want to lose this info!

Harddrive slow all of a sudden?
No processes running that are hogging your CPU?
Then the problem could be that the harddrive's data transfer mode has gotten stuck in PIO mode.

To confirm this (this is in Windows XP):

• Start | Settings | Control Panel | System | Hardware | Device Manager | IDE ATA/ATAPI controllers
• rightclick on each of them, select properties, then Advanced Settings tab
• see if any of them have Current Transfer Mode = PIO

If so, here is fix (this works on Windows XP)

• Backup the registry
• Open the registry editor
• Expand this path
  Code

  

  HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E96A-E325-11CE-BFC1-08002BE10318}\000X 
  
  

• The last four digits will be 0000, 0001, 0002, 0003, and so on.
• Click directly on each of the numbered folder/keys, then delete all occurrences of the following values on the right side
  Code

  

  MasterIdDataChecksum
  SlaveIdDataChecksum 
  
  

• Reboot.

Then go back and look at the IDE devices controllers again and confirm that none are in PIO transfer mode.