Do you have any other AV program installed on the machine? Have you ever had Norton installed on the machine? Do you use firewall? If yes, which?
90% of such troubles are caused by conflicting low-level programs. Unfortunately, these are usually very hard to resolve.
To work around the problem, you need to boot into the Safe Mode (by pressing F8 before the start-up XP screen appears) and uninstall avast from there.
Sorry - and please give us the answers to the questions above.
Thanks
Vlk