microsoft GS wavetable synth, as far as I know, is a general-midi unit which has the simplest of simple general midi collection on it. It is not, therefore, related to recording audio as such and shouldn't be worried about unless you want to use the sounds it contains.
If you have asio4all selected in mixcraft it should work straight out. Normally you turn off the windows sounds in controll panel as Sea Monkey says, but if it is just the built in soundcard you are using it may need to be on... as he suggested, check the forums, as asio drivers on stock soundcards is not an area I have ever really used (except in when I first got started, and it all worked so I never had to troubleshoot!)
To R2RO, are you using these vst's as standalone applications (in which case, you are not using them as vst's, a vst is a plug in that works within a host application). I don't think the Wavetable synth would effect the apps, in fact, it should have nothing to do with a vst/standalone vst thing. I had the wavetable synth as an option in cubase for years, and it worked if i selected it as the 'plug in' for the midi track, though I never used it, using third party vst's instead. It never got in the way.
However, maybe it is associating the midi signal coming in with the wavetable synth? I can't see how that would effect a vst listening to the same midi channel, but try turning it off anyway..
in both cases, buying a cheap but 'proper' audio device would more than likely solve the problems.