hey and welcome to the avast forum. im using the latest avst i have no poblem with this at all. but i can be something with the new added feture in the latest build that gives you problem (stremfilter) you can try to disable it throught the add/remove program uninstall avast and add/remove compontet from avast. did this solve the problem?
second you should consider upgrade windows xp to win 7,8, or 8.1 seens there is no support from windows xp anymore. ans sp3 forxp been out for years 