It could be anything, but is probably unrelated to the slice tools. The WED mouse functions were not changed in ages. Only in the last version, the mouse sensitivity setting was added due to your problem. But this apparently did not affect the problem. So the only conclusion that I have now is that it's some very strange mouse driver bug that for some reason only happens on that one PC and nowhere else, and only in combination with WED.

Some mouse drivers allow to activate different mouse settings depending on certain applications - is it possible that you have activated such a setting for the application "WED.EXE"?

Otherwise, the only suggestion I have is that you install some different, generic mouse drivers, such as the Logitech mouse driver. Or buy a new mouse. Buying a new computer, I guess, would be sort of overkill.