Dumb questions first:
- What mouse sensitivity have you got set in the game options?
- Have you got mouse acceleration/smoothing (can't remember which it is) on or off in the game options?
- What mouse speed have you got set in Windows Control panel?
- What mouse are you actually using, and if a gaming mouse, what DPI have you got it set to?
When I'm at home later I'll post my settings for the above if it helps as I haven't noticed any issues at all in Bio2. Sure someone'll beat me to it and post their settings first though.