Windows 7 Home Premium 64-bit. Running Avast antivirus. The machine has been running fine for several months. Last night, I noticed the screen blank, only the mouse was visible. So the Windows shell explorer.exe had crashed. I did Ctrl-Alt-Del > Task Manager > New Task > explorer.exe and it came up for a few second and then crashed again. Rebooted, got the login screen, and within a few seconds explorer crashed again.
After a lot of googling, I came across a tip which said that explorer.exe was missing from c:\windows\system32 but it was present in c:\windows. So simply booting in safe mode, opening a (Run as Administrator) command prompt and copying explorer.exe from c:\windows to c:\windows\system32 and rebooting fixed the problem.
Question: What would cause this to happen? Anyone seen this before? I ran some malware and antivirus scans but nothing came up. Any idea what's going on?