There was a documented error in a SP2 release candidate that
exhibited that behavior. According to MS, it was a problem with
the "Acceleration" function of mouse control. However, it was fixed
for the final release of SP2.
Is this a traditional "Ball" mouse or an optical one ? If optical they
are sensitive to the surface you mouse on (?). Try a different pad or
surface. To test, rest the mouse on a White sheet of paper and watch
for the mouse pointer to move on it's own.
Depending on which vendor mouse you have, you might check for
updated mouse drivers which might help resolve the issue.
Microsoft Mouse products has a new Intelipoint package at 5.2
Logitech Mouseware hasn't been updated in a good while, but I
believe their current package is 9.79
Microsoft has a Tech note on how to resolve this type of problem,
but it requires some detailed work in the Registry. Check the symptoms
carefully as this may not directly address your specific issue.
http://support.microsoft.com/default.aspx?scid=kb;en-us;321122
Don't undertake this process, unless you have a System image backup.
Try the other troubleshooting measures first and repost your results.