Details: Win7 x64, HP desktop tower, nothing special.

Frequency: Have been using this computer for >2 years, but this only started today after rebooting for an update, yet forums several years old suggest this problem is not uncommon. Some even report this is a bug from the Win 3.1 days (seems unlikely). Over the day, behavior has subsided and returned multiple times.

Antivirus is in use, have scanned multiple ways, no hits. No suspicious services or processes.

Tried: rebooting, different mice, ctrl-alt-del, holding modifier keys.

Holding modifier keys seems to temporarily fix the problem.

I'm posting this in hopes someone has come across specific causes. Assume I've done all the typical stuff, event logs don't reveal anything, and there are no custom devices or drivers in use.

Going crazy!