‘Virus Chest’ where stuff is quarantined:-
https://blog.avast.com/2015/03/19/how-to-use-the-avast-virus-chest/
If restoring the files from that doesn’t work:-
Are there any pre-AVAST update or pre-scan Restore Points available? Using Restore Points is one of the main purposes of Safe Mode?
It is considered good practice to put in a Restore Point manually before installing or updating ANY program.
Most good programs that may affect/interact with Windows system files put in a pre-update/install Restore Point by default. With AVAST I don’t know if it does this now but I do know that at one time it did make using Restore Points more difficult than it should be. I won’t comment on that bloody stupid decision. Oops. 
Anyway using a Restore Point can, usually, be reversed if it doesn’t help.
In that case then I’d try a scheduled Chkdsk repair scan which should run on the next boot and fix any indexing errors that might be causing the problem.
If neither that or the Restore Point solution works and you have a recovery partition this maybe the time to use it. Alternatively, if you have the original XP installation discs available, then as a last resort I’d try running System File Checker to repair whatever damage has been done.