Avast Anti Virus is a FREE product and can be downloaded from HERE but does require registration each year.

To register the product after installation and to renew it annually, follow the steps below

Step 1. Locate the Avast icon on the task bar.
Step 2. Use your mouse to right button click the icon.
Step 3. Select "About Avast".
Step 4. Click on Licence Key.
Step 5. Click on Program Registration.
Step 6. This will redirect you to the Avast Registration Page.
Step 7. Enter your details including your email address.
Step 8. Avast will email you a new Registration code to the supplied Email address.
Step 9. Once received - highlight the code and copy and paste into the licence key field at Step 4.

You are now registered for another year of Free Anti Virus protection

The procedure of inserting the key into the program is shown in a video at: -
http://www.avast.com/files/tutorials/insert_key.htm.