If you receive an email with the subject "antivirus support" telling you to download AVG Security 2009, DON'T click on the link. THERE IS NO SUCH THING AS AVG 2009 (sorry for shouting). This is a NASTY piece of spyware that will keep popping up messages on your computer.
If your computer does get infected messages will keep popping up telling you how many infections have been found etc. You will need to run genuine antivirus and/or antispyware software to rid yourself of the infection. The two pieces of antispyware software I recommend are Adaware from Lavasoft (beware of spyware laden imitations - click on the link for the real version of Adaware) or MalwareBytes.
If you are an AVG user, the latest version is AVG8. It has no connection with the fake 2009 spyware.
To prevent your computer from becoming infected with viruses and spyware you should install effective antivirus and antispyware software. One very effective and highly respected internet security suite is F-Secure. This protects you against viruses, worms, trojans, spyware, hackers and spam. It also helps protect children from unwanted internet content.