There is a rash of virus's going around. Five recent incidents among friends and family. I mentioned this before, but can't remember who I sent it to, so new warning.
One of these problems consists of popups that purport to be warnings of trouble on your computer and want you to buy their fix, or do a scan, etc. Something along those lines. The popups continue and get worse with time, and can make your computer unusable if not caught early.
A variation has a popup that says "Windows Explorer has stopped working", and gives you a choice of shut down or restart. Choosing either gives a second popup attempting to restart. This doesn't work and the popup continues. Nothing else works right.
You can get rid of the first one with Spybot - Search and Destroy if you run it early enough. Spybot is a free download.
http://www.safer-networking.org/en/home/index.html
If this doesn't work, more drastic methods may have to be used.
The second one may also respond to Spybot, but maybe not. If not, try a restore to a previous date.
A scan with your anti-virus may work, but will probably have to be done in safe mode on a re-boot.
I would be interested to hear if this is a general problem, or just local to my area.