Resident protection (the real-time protection of the operating system), is one of the most important parts of an antivirus program today. avast! features a powerful resident module that is able to detect a virus before it has any chance to infect your computer.

avast! Home Edition contains resident protection of the computer file system and a resident module for e-mails and news.

File system protection ensures that no virus will be started on the computer. It offers a wide range of settings, such as the possibility to specify that files will be scanned during copying, or that the scanning will include files with given set of extensions only.

E-mail/News protection consists of two independent modules; first, there is a generic scanner working on the SMTP/POP3/IMAP4/NNTP protocol level. It is capable of protecting any existing e-mail client that uses these protocols. Second, there is a special plugin for MS Outlook only; the mail scanning is completely transparent, requiring no special settings.

A new feature of version 4 is heuristic analysis of e-mail scanners. This feature can protect against new, unknown viruses and worms that are not possible to detect by the usual means. The heuristic module performs a thorough investigation of every e-mail message and watches for suspicious signs, that might announce virus presence. When the number of those signs exceeds a user-defined level, the message is considered dangerous and the user is warned.