avast should startup automatically when you boot your computer and scan for viruses. you should see the little Circle A spinning in your system tray. Just make sure to update it on occasion and you should be golden.
did you happen to register it?
you have to register or it stops functioning after 30 days or so. dont worry its FREE and they send you the key in your email.
You can also check out Microsoft's Windows Defender for spyware protection if you want something scanning all of the time to protect you (spybot and the free version of adaware do not do this)
http://www.microsoft.com/athome/securit ... fault.mspx
It does a great job keeping spyware off your system and its free as well. This combined with avast makes for a pretty clean system.
(if you are running windows vista, this is preinstalled)
and dont forget to use Firefox as your browser, this alone will help you avoid 90% of the problems out there
Other than that, update windows every month to get critical patches and you should be alright