If you have Microsoft Office installed this may be the reason. Check =
your Startup group. When you install Office (I think it's Office..maybe =
it's some other MS product), one of the things it installs is a program =
called Fast Find. This indexes the files on your drive so various =
Windows programs can look stuff up faster. It's put in your Startup =
group, but there is never any icon when it's running, so you generally =
don't know it's going. By default it is set to index every four hours. =
It's possible this started running during your second block.
Shaun