How to disable MS GS WAVETABLE synth? or what is occuping it?

I fixed a similar (possibly the same) issue by disabling all system sounds in the "change system sounds" settings dialogue - which is a good idea for anyone doing music production on their computer. You can get there by right clicking the sound icon in the system tray, choosing playback devices, then clicking on the "sounds" tab on the top. Under "sound schemes", select "no sounds." This prevents the windows os from accessing the default midi definition file (MS GS WAVETABLE SYNTH), which seems to be your issue.

If that doesn't work, you could see if your driver has a settings dialogue that allows you to disable the midi features of the drivers.

Does your midi controller support it's own midi definitions? You are likely to have more success if you use something besides Microsoft's very generic sounding midi table. If I'm not using my midi keyboard, I typically run my midi through a plugin like SFZ so I can control the midi output format. You can download free midi banks and soundfonts on many different websites.