Hi fusilero,
This is a little strange because people who have SP2 installed, report avast is recognised, so there may be another issue here.
What version of XP do you have? Apparently XP Pro users can't currently update in the regular windows update, yet XP Home users can. Strange since they have paid a greater premium for Pro, perhaps MS think all Pro users are businesses and will have been able to get the update through an other update source.
If you have broadband I would suggest that you download the network install at about 266MB ish, rather than do the online update. This gives you the opportunity to save SP2 to CD, etc. so should you need to format your system and re-install XP you won't have to download SP2 again.
Check out some of the other SP2 threads in the forums, many have links to various download locations.
HTH David