My PC randomly just sat there with the hard disk light on but I wasn't doing anything - plus it was dog-slow.
Using the excellent Process Explorer, I discovered that the "cisvc.exe" program was the cause of the problem. This is part of indexing services.
My first solution was simply to suspend this via Process Explorer. (Right click on cisvc.exe). This helped but I had to do it every time on start-up.
So I googled and found one possible solution:
My Computer - right click on the C: drive - Properties - uncheck "Allow Indexing Service to index this disk for fast file searching" in the General tab.
This helped but the problem still remained.
Then I found another, better solution:
Control panel - Administrative Tools - Computer Management - expand "Services and Applications" - right-click on "Indexing Service" - "All Tasks -> Tune Performance". Then decide what level you want.