I have also been using BeSweet with the GUI ver v0.6b73 for some time now and on Win2000 OS without ever a glitch. AC3Machine works fine also.
What is the actual version of BeSweet (not the GUI) you have installed ?
If you having probs with both BeSweer GUI and AC3 Machine, then I would suggest your copy of BeSweet is very old or corrupted. Get the latest from
http://www.doom9.org/index.html?/software.htm in the Audio section