Hi,
Thank you for posting in Symantec community.
Symantec is still supporting Windows 2000 machines with SEP 11.x version.
I would recommend you to upgrade Windows XP machine to Windows 7 & use SEP 12.1 latest version.
In the following KB article this is important para to read:
http://www.symantec.com/docs/TECH204937
"After the official End of Support date from Microsoft goes into affect, Symantec Support may not be able to provide full threat resolution on XP systems due to a lack of Microsoft security patches. It has always been Symantec's stance that proper network and workstation security begins at having the most recent security patches from the OS vendor. Depending on the nature of the threats involved, Windows XP may be susceptible to viruses and risks that more recent iterations of Windows is immune to."
Check this KB to find SEP End of life cycle dates:
http://www.symantec.com/business/support/index?page=releasedetails&key=54619#