Grisoft' current AVG Free 7.1 anti-virus software will be discontinued on January 15. However, this doesn't mean that AVG Free Edition will go away (though some people seemed to interpret this way). Their new 7.5 free edition is available now from...

AVG Free Advisor

The page also links to Grisoft's free Anti-Spyware software.

According to the information on the page, version 7.5 works with Microsoft Windows Vista (7.1 did not install when I tried it with Vista a few months ago). Note, that this Free Edition is only free for personal use, not business use.