I usually get the Windows Explorer error when there is alot of physical memory being used. Knowing you had Vista until you got 7, is there a error-checking option in the main (C:\) disc? That may help.
Please don't say you thought it was a virus and you need to back up files...
Okay this happens if you have a lot of stuff open for a long time and you open up something else... OR if you've just opened up your Computer and tried to access files immediately... or it just happens randomly like it does on mine. It doesn't affect files since it's only the explorer.exe getting slowed down or being unresponsive.