https
/antivirus.comodo.com/
However, you need to upgrade to a newer OS if you're concerned about online security & malware (and you should be concerned now more than ever).
More and more AV software developers have either stopped supporting XP or will do so in the near future. It's on borrowed time, well and truly. It's fine as an OS not connected to the net, but otherwise it's risky to use in my opinion.