There's a virus that locks you out of windows, that's going around in Russia. My father-in-law managed to get it (don't ask how), and Avast! did not pick it up. I have sent it to Avast, so they can put it in the next update.
The file involved is gftkcydl.exe, and it shows up on on Virustotal as having 5 detections.
It causes a very large red and white banner to show up and block most of the screen. This banner tells you to send an SMS to a number, which then charges you (reports claim it's about $10 a message), and send you the number to put in to unlock it. This will repeat itself everytime you restart until the virus is removed.
Housecall was able to remove it for me, so if you get it before it's put into Avast's definition, I'd recommend them.