free antivirus software which don't have to pay for

1. AVG is a free virus and spyware removal tool.
This tool includes an e-mail scanner, link scanner, scheduled scanning options, automatic updates,
and more.
It will work on Windows 7, Windows Vista, Windows XP and Windows 2000.

2. Avast
Avast is another choice of anti-virus that supports Windows 7, Windows Vista, Windows XP and Windows 2000.
This program has a fully functioning anti-virus and anti-spywares engines protecting your PC from threats from the Internet, your files, emails, and even instant messages. Also, this program require you to register free key code it in order to use.