I've had documented problems w/ SoftIce with *any* mouse on some mainboards. It may be due to non-standard hardware; it is quite possible that SoftIce's drivers do not have the robustness of the WDM drivers (likely, too), and MS mice have changed significantly in recent years wrt actual protocol. Have you attempted to use a basic, two/three-button mouse and/or a PS/2 mouse? YMMV.

Alternatively, it is absolutely possible to forgo mouse usage altogether. More to the point, I believe that SoftIce's USB support is almost nonexistent, breaking readily.

-archaios