Dude I have your EXACT problem... logitech optical wheel mouse, mouseware version 9.75. What's weird is that sometimes if i open a browser window my sensitivity increases a lot, then i can minimize the window and as i return to the desktop my sensitivity goes back to normal. I can then open AIM and sometimes my sensitivity will increase a lot, then as i open a different window it will decrease again.
Its REALLY frustrating because when i load up CS sometimes my sensitivity will be how i like it, and other times it will be tremendously slower causing me to have to reboot in order to fix the problem. It's the mouseware i think, i read on some other thread that you should just go with windows default drivers instead of logitechs:/ Let me know if you get your problem solved. By the way, im running 98SE if that makes any difference.