We had a similiar case and we found lnk files pointing to the use of CCLeaner, however it on its own is rather harmless, however if you look at the SOFTWARE hive you, there might be a folder Heidi's which contain the Eraser.dll this used in conjunction with CCleaner can potentially cause this anomally.
Have you found anything on searching for CCLeaner and Heidi's Eraser.dll as any lnk files or keys in the SOFTWARE hive could point you in the right direction. If you do find this this would explain the missing USBSTOR
1000 PreFetch files in XP? Really? I seem to recall that when XP hits around 128 Prefetch entries it starts replacing on a FIFO basis ……………………
Also, just the presence of CCleaner or Heidi Eraser doesn't prove much unless you can get hold of configuration and log files. Which may be difficult.
Even if these apps appear in PreFetch or UserAssist, it doesn't mean that they were actually run 'in anger'. Could be (we had one instance) that a user downloaded CCleaner, installed it, opened it, then decided it was just too complicated for them to understand, and they never ran it again. No harm done.
Cheers