Please read:
Support for Windows XP Service Pack 2 ends on July 13, 2010
http://support.microsoft.com/gp/lifean31

Sticking with XP SP1 will not help you and avast! needs at least XP SP2 for support.

What are the supported operating systems for avast! 6.x ?
https://support.avast.com/index.php?languageid=1&group=eng&_m=knowledgebase&_a=viewarticle&kbarticleid=750#idt_01