I've had a similar problem: I once plugged in a PS/2 mouse, but switched back to my USB mouse.
I'm almost sure you mean the mouse in XFree, so have a look at /etc/X11/XF86Config-4
Option "Protocol" "IMPS/2"
Option "Device" "/dev/usbmouse"
Option "ZAxisMapping" "4 5"
This is what my InputDevice section looks like.
Yours, at this time, would look a bit like this one, but that's not what you want, ofcourse.
I'd say, first run (the text mode) drakconf
, then configure your mouse the right way with it.
If that doesn't work, check the option "Device" and set the "Protocol" to "auto".
And if that doesn't work, or you aren't very content with having to move your mouse briskly up and down everytime you start X, (like I had to) post your XFree86Config.