You have to install the ‘Restoring Console’ (poor translation) as boot option in XP. Look (search) in the help for ‘console’ and you will find the exact command for it. It’s not like the safe mode but it’s the best of XP has.
Other option is pressing down the F8 key while booting and you will see a lot of booting options… Please, don’t do anything if you are not sure… Changing the XP boot could let you in ‘hot water’ 
Here you have a lot of avast! ‘hidden’ configurations: http://www.avast.com/forum/index.php?board=2;action=display;threadid=1647
Disable the setting ‘Show detailed information…’ (I’m trying to translate the meaning of the option…) at the Advanced tab of the providers settings. It’s uncheck by default but maybe you have check it sometime ago…
Hope this help. 