|Just searching for "spybot free" gave me no advert for cyber defender, nor did clicking on any of the normal non-sponsored links that I tried.|
You should never download unsolicited software - always check credentials. SpyBot is quite safe.
I use both Avast & AVG (different machines) and have found both of their current versions well capable of alerting me to viruses whilst online. Neither have ever let anything through. No AV is perfect, there is always a chance that different virus checkers are better able to deal with specific viruses than others. I would generally rate both as good.
some other bloke...