"There are only two programs needed to keep any computer clean forever" - hooray for wide and blanket staments like these!
Im just wondering how <insert name of anti virus software> goes about detecting the ones that <insert same name of anti virus software> has been proven not to detect...
One program is not good enough.
Im happy with Zonealarm so I can control what is going
out of my machine, AVG for virii, Spybot for malware, Firefox with No-Script plugin and plain old common sence.
"All the other default windows security is fine" - No it isn't. The Windows Firewall has numerous flaws, only tracks what is coming
in to your machine, and is known to have backdoors in it. Have a browse round some of
these links.
To be honest, common sence is your best friend here. Don't open attachments from people you don't know, always have "Show known file extensions" ticked (so you can see that "sexypictureoflady.jpg" is actually "sexypictureoflady.jpg.exe") and don't go browsing the dark places of the web, and keep your Operating System updated and patched.