Hello,
I would suggest you to please restart the XP machine and work on the Manual uninstallation steps provided in the Article below:
How to manually uninstall Symantec Endpoint Protection 11.x client from Windows 2000, XP and 2003, 32-bit Editions
http://www.symantec.com/docs/TECH102261
How to manually uninstall Symantec Endpoint Protection 12.1 client from Windows XP and Windows 2003 32-bit and 64-bit operating systems
http://www.symantec.com/docs/TECH163585
Hope that helps!!