Hi,
This may sound like a strange recommendation, but Microsoft has been doing some strange things lately with the MIDI support in Windows.
Please try installing the LoopBe1 that you can find free at
http://nerds.de/en/download.html make sure it is
LoopBe1 (free) and not the LoopBe30 (pay for). We have had one customer who reported that the GS Wavetable was missing in any configurations until he installed the LoopBe1.
It's easy, free, and so worth a shot
Looking forward to what you find.
ttfn,
Sherry