Windows 7
Microsoft built in Firewall
Microsoft Security essentials
Been using this free virus scanner from microsoft, along with windows built in firewall, for a couple of years, never had any problems.
I use Ccleaner to clear out any junk.
Microsoft must have a better idea than anyone of what to protect as they designed the operating system and they are hardly likely to put a rubbish firewall and virus scanner to protect thier operating systems.
You dont need third party security the above is all Free!
Megawatt91