Before re-installing anything,
you should start in safe mode and try to troubleshoot the problem.
On Dell computers, you simply have to press F8 when the computer is starting.
A menu will offer you to start in safe mode.
Are you up to date in terms of Windows XP security updates?
If no, you should start there.
http://windowsupdate.microsoft.com
Did you try with another mouse?
If you don't have another mouse, did you look on your mouse provider's help center if there is such a problem described and solved?
Is your mouse driver updated?
Do you have an anti-virus?
If yes, please run a full scan.
If no, you can install a free anti-virus like Avast, Microsoft Essentials or Avira AntiVir.
http://www.avast.com/free-antivirus-download
http://www.microsoft.com/security_essentials/default.aspx?mkt=en-us
http://www.free-av.com/en/index.html
Please provide any information you can. It might help.
* mouse model
* when did the problem start?
* what were you doing/what was the last program installed?