I'm NOT currently on my Vista machine, but I have one instance of Explorer.EXE running. (XP-Pro-SP3)
Are both instances exactly the same, spelled the same and everything?
Can you close either one without crashing windows?
OK, I fired up the Vista PC and checked it out.
It has just ONE version of "explorer.exe" running and it's all in lower case.
I'm not running any other programs, but Vista. I don't even have an AV program installed on that PC yet.
Have you scanned your PC for Viruses and Trojans?
My first guess would be one of those.