Resolved Explorer.exe Unstable

Within the last few days, this process (part of Windows Explorer) has dramatically slowed down my machine. I can be totally idle (normally CPU is maybe 2% at idle, explorer.exe is zero), but now explorer.exe oscillates between 25% and 75-90%. The CPU usage history looks like a wildly vibrating spring. Have checked for malware, AV, event log, msconfig (startup list..3), service list, etc, but I can't figure out what's stimulating explorer.exe to grind away. I did add one of last week's patches from M$, and a new cable modem, but I have no reason to suspect either of those. Any ideas about what explorer.exe might be doing? XP/SP2

 

Solved it.....boy, do I feel dumb. Unknown to me, my grandson plugged his digital microscope into a USB port on the back of my PC. Explorer.exe must have been looking for it, it wasn't showing up on my monitor for some reason. Aaargh! Sorry.