If this were an older operating system, I'd immediately think it was a DLL that didn't get rolled back with the main driver. More than once I had to manually extract and and replace DLLs for my weird-ass Swiss keyboard/controller (the Swiss may make great watches, but they make really crappy software). System File Checker wouldn't flag the difference. I don't know if that's an issue Vista has, though.

Also, are you leaving McAfee running while playing L4D? With Vista I often had problems with the antivirus deciding to do something in the background and the whole kit locking up as a result. This was especially true if I was doing something online.