AVG Anti-Virus
AVG also comes in two flavours, the Free and Professional edition. Like Avast and AntiVir the Free edition may also be used within the private sphere and for non-commercial purposes.
Since there's no need to further discuss what a scanner is all about I'll explain the main differences between the Free and Full edition

AVG Free: download
Latest virusdefinitions: download
License / key: 1yr license
System requirements: Windows 98 or Windows 95 with Internet Explorer 5.01
Features:
- Realtime protection: files will be scanned when you download, open, save/use them)
- POP3 (email) scanner which scans all incoming and outgoing messages and attachments
AVG Professional: download
Latest virusdefinitions: download
License / key: 1yr license
Key differences between the Free and Professional edition, though the Free edition offers the essential protection.
- Multilanguage support ( English is the only available language in the Free edition)
- Flexible scanning and updating options, where the Free edition only allows one/day
- Exclusions of files/folder/extensions
- Customizable scan options like selecting certain folders to be scanned for viruses.
Installing and configuring AVG PDF
AVG problems and solutions