If you are using Windows XP, why don’t you schedule an avast running at boot time? You will be able to solve what you posted first: some virus locate themselves in the memory during the Windows startup, then most Windows-version antivirus softwares can’t kill these virus, the only way to delete them is to boot the computer from floppy drive (or other removeable drive) where there is a DOS-version antivirus software, that is, only in the DOS environment, the virus can be cleaned up, not in the Windows.
You can make a bootable CD with avast for DOS or, better, you can give a try on Bart CD from Alwil Software. 