I have the same problem and I am POSITIVE it is not a virus.

I know for a while my fingerprint scanner and software was causing it to come out of standby mode, but now that I have fixed that problem, the explorer.exe problem sitll remains.

It will go to the maximum of both cores if ANY explorer window is open (except the desktop itself) for too long (30 minutes is more than enough).

My memory use though always stays at around half of the 2GB though. (for all of the processes, not just explorer.exe)

I hope someone fixes this soon, it is really beginning to hack me off!