With Vista 64bit you have 2 WMP11 executable files, one for 64bit and one for x86.
Lets try and revert your WMP11 back to x86 mode as it is more than likely in 64bit mode at the moment and this could be causing you the problem. You will not see any difference with WMP11.
Check out
Switch, Change or Set 64-Bit Windows Media Player 11 (WMP11) in Windows Vista x64 as Default » My Digital Life for information on how to do this.
regards
Steve