when I do boot scan , every file being scanned has error 42100 at end of file. Does anyone know what this means. I have XP Pro installed. 
Would this help maybe?
Yes this link helped, I tried doing search for error 42100 but notihing came up, so thought there was nothing posted, but guess I was wrong. I am also runing XP PRO (SP2) with AMD64 CPU.
Once again thanks for quick response. 
Strange, this does noe match the list of avast errors that I get with pk ;D
Must I change it? http://forum.avast.com/index.php?board=9;action=display;threadid=4818;start=msg65672#msg65672
It is correct. It is indeed this error:
42100 AVAST_POLYVIR_EXCEPTION Exception in polymorphic viruses code
What proves, for me, that numberer error messages make no sense ;D
At least, the solution seem that does not concern with the problem. Of course, there is a reason but the user will never understand the error message.
Sorry, this is not avast related, all error messages take this non sense way