AntiVir
Like Avast!, AntiVir also is a scanner which comes in two flavours, the Free edition, PersonalEdition Classic, for home and non-commercial use and the Premium edition.
Since there's no need to further discuss what a scanner is all about I'll explain the main differences between the Free and Premium version

AntiVir PersonalEdition Classic: download
Latest virusdefinitions: n/a
System requirements: Windows 98
Features:
- Realtime protection: files will be scanned when you download, open, save/use them)
- Protection against trojans, worms, backdoors, jokes and other harmful programs/spyware
AntiVir PersonalEdition Premium: download
Latest virusdefinitions: n/a
Key differences between the Free and Premium edition.
- Adware/spyware scanner is included, not really necessary since a lot of high quality freeware adware/spyware scanners are available
- POP3 (email) scanner, which scans all incoming and outgoing email, and attachments, for viruses. Now there's an option very usefull, but a lot of freeware scanners have this option built-in
- Customizable scan options like selecting certain folders to be scanned for viruses.
Install and Configure Antivir Free PDF
Problems using AntiVir