Maybe there is a new malware which affecta only modified copies of explorer.exe maybe exploiting some bugs.
Unlikely. This is easily reproducible by anyone with a 64bit W7 machine.
1. Install Avast! & lastest definitions
2. get W7SBC and a random bitmap file for that (plus gaining control over explorer.exe if needed)
3. Once the changes are applied, Avast immediately blocks explorer.exe
4. If the changes are unapplied, it is immediately unblocked and if the changes are reapplied, it is immediately blocked.