"If you already have Microsoft Security Essentials installed on Windows XP, you'll continue to receive antimalware signature updates for a limited time
Which means eventually MSE on Windows XP will be next to useless.
Avast Antivirus is the only other free one which will run on Windows XP (SP2 or SP3 required): http://www.avast.com/index
Eventually, however, Avast updates will also be unavailable for XP users as the application is improved and new features added which XP doesn't support.
You really should consider moving up to a newer version of Windows so you'll be able to use any antivirus that's out there.
I use the free Comodo Antivirus & would recommend it to anyone without hesitation, but it requires Windows Vista or later.
message edited by phil22