I found this which might contain useful information to you:
I do not have Windows XP but I do use Avast and I like it, but then again, I have only ever tried 2 anti-virus software...
You should have an active Antivirus and an active Antimalware program.
I am no security expert so I'm not sure which ones to recommend to you, but I have heard good stuff about Malwarebytes Anti-Malware and Emsisoft Anti-Malware (Which Emsisoft can be used instead of an Anti-Virus)
I have only tried Malwarebytes FREE so far, PRO gives you active protection while FREE is just a scanner. Same with Emsisoft.
Maybe as an extra security layer, you should have something like Malwarebytes Anti Exploit BETA, I use it and even though it has not blocked any exploits for me (Because I don't think I have encountered exploits yet because I am very careful).
It says Windows XP is supported BUT it is BETA so only use it if your comfortable with that.
It does not protect agains Windows Exploits but it does protect these programs against exploits:
Java, Flash, Shockwave, Acrobat
Adobe Acrobat Reader
Adobe Acrobat PRO
I hope that helps...
I also found this that has some tips to making your XP machne more secure:
Edited by I have a computer..., 28 April 2014 - 10:11 AM.