Quite honestly, Vista was the most secure OS out of the box at release of any OS ever released, including Mac and Linux.
However, a good AV is good to have, but firewall or so called internet security AV, unless it is a hardware firewall far more recommended, is really not needed by windows as of XP SP2.
Windows Firewall, while not perfect, is as good as most software firewalls on the market. There are plenty of articles on the net to backup my claims.
For Malware/Ad-Ware/Spyware, really the best progs are Spybot S&D and Malwarebytes.
I don't promote Ad-Aware anymore cause it is bloated and overly commercialized unfortunately.
I don't promote NOD32 anymore as it cripples the system when the license expires (it's a good AV though), and I just do not approve of this tactic at all. I do not know what else it cripples, but the ability to save pictures from the net is one of them. Uninstall an unlicensed NOD32 and saving pictures works again.
Kaspersky is still not 7 ready, it's a pain to install. Parts are not compatible unless you install a patch (possibly) from kaspersky to fix it. This shouldn't be needed.
AVG is junk.
Avast is ok.
Mcaffee is junk
Norton 360 can at times choke the system, or cause the network to drop randomly, caused by it's Internet Security junk.
I have not yet tested Avira, mainly in the past they did not accept US currency, now they do, so I may check them out further. They always get the best ratings on AV Comparatives. But, they also force you to contribute to Auerbach Foundation which I need to research. I don't like to contribute to charities I know nothing about.
I use
AV Comparatives as a basis for what is best AV. But also, my personal experience over the years on my own pc and many others.