It may be something to do with Disk Defrag.
I have read several blog entries that recommend unchecking the:
'Increase task priority, if three consecutive runs are missed'
box in the 'Optimise Drives' settings.
I have just done this on a new install on a laptop that was previously affected.
The laptop is running fine at the moment , but insufficient time has elapsed since the setting change to prove that this is the problem.
The jury is still out.
If this fails I think I will install LINEX. 🙂