Fake Security

I have had a fake security on my PC fo the past 2 days which would not let me access the internet and kept redirecting me to buy vista internet security, luckily I am not gulliable and didn’t fall for this scam which I later found out from visiting various forums that this site actually nicks the money from your bank account after you panic and enter your bank details to buy the security. I finally got rid of it by doing a system restore which I had to do very fast 'cos this fake security stops you from accessing some of your programmes.
Anyway, my question is why did avast not block this malware!!!
And when I scanned using another login account did it not pick it up and delete it or move it to the virus chest!!!
Can anyone answer these questions for me?

What ever any kind of anti virus you will try. They wont be 100 % virus safe. avast! has a good detection again virus. He is about 98,99 % if im trusting the anti virus tester.

Anyway, my question is why did avast not block this malware!!!
No security program have 100% detection and new installers for these rogue programs are released daily/weekley