We got around it by moving the mouse out of the drawing window also. The funny thing is we didn't have to do this until we migrated to Windows 2000.
Lucky thing is we don't have to worry about fractions, we work in mm.
What seemed to work for a while was to re-create a users entire windows profile, but after some use the problem returned. May have something to do with the fact that medusa is not a true windows application. I much prefer my unix install of medusa.