I seem to remember this came up in another thread awhile back, and it ended up having to do with there being a large number of downloaded, but not yet installed, Windows Update files (for those with settings "download, but let me decide", if I remember correctly).
The thread was started by Harry....if I can find it, I'll edit this reply with a link.
EDIT: Okay,
the thread I'm thinking of was about the computer completely freezing up when Calibre started. You might want to look this thread over, there were lots of suggestions, including the one to clean out the Windows Update files (which is what I had to do to get my problem to clear out).