I have installed on many computers Avast Antivirus Free.
Apart from the fact that this is a free product, Avast offers multiple layer of protection (including Web Guard and Site Advisor), it's very user friendly, and will not slow down your computer.
Here are the 3 main features that would make me choose Avast over Avira or Microsoft Security Essentials:
1. The avast! Auto-Sandbox
The avast! Sandbox is a special security feature which allows potentially suspicious applications to be run automatically in a completely isolated environment with no risk to your computer or any of your other files.
By default, if an application is started and avast! detects anything suspicious, it will automatically run the application in the Sandbox.
The advantage of running an application in the Sandbox is that it allows you to check any suspicious applications while remaining completely protected against any malicious actions that an infected application might try to perform.
The browser or other application will then open in a special window with a red border, indicating that it is being run inside the sandbox.In the “Browser Integration” tab, you can further specify that your browsers should always be run in the Sandbox to ensure you are always protected while surfing the web.
When the Sandbox is closed, it will be restored to its original state and any downloaded files or changed browser settings will be automatically deleted.
2.Avast Reputation Services
Avast File Reputation Services will check if a file is safe even before it is opened, by checking its database of known files. Whether the file is considered safe or not will be determined by how many other users have the file, and for how long it has existed.
3. Avast Software Updater
Malware often spreads by exploiting known security issues in commonly used software.
This means if your software is kept up to date, most malware would be unable to spread!
Avast Software Updater shows you an overview of all your outdated software applications, so you can keep them up to date and eliminate any security vulnerabilities.