If it is a ball-type mouse have you tried cleaning it? Turn it upside down, twist the circular plastic around the ball to unlock it, remove the ball and clean it. Then look inside the mouse and check if there is any crud on the rollers - if so, carefully scrape it off. Then reassemble.
If this is not the problem, check your mouse settings on the pc as it will give various options for adjusting the speed and movement of the cursor. One option, for example, forces the cursor to automatically jump to an active dialogue box.
Just a couple of thoughts. If you have an optical mouse and do not use a mousepad, that particular brand may not respond well to "refelctive surfaces" such as high gloss finishes, so maybe try either a mouse pad, or a piece of paper under the mouse and see if it helps. Also, you may need to clean the area around the optical light as sometimes it can pick up debris from the desktop. If a ball mouse, Surreal has already explained. Also, you may have jiggled the connection loose moving the mouse about the desktop. You must shut down the computer to see, if connected via PS2, turn the computer off, remove the mouse, reinsert and then reboot. Hope one of these might help out.
when did the problem start? Have you installed any software that may have caused it?
If none of the above steps helped, try going to start, run and typing "msconfig" without the quotes and hitting enter. uncheck all the boxes and reboot. If the problem seems to be fixed, you have a software program causing you issues. If you have a second mouse try it as your mouse could have died.
Spyware or a virus could cause this also. I would suggest scanning your system for both.
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