The intrusions are most likely tracking cookies or best known as "data miners" sites use to follow your browsing habits. The security options in the browser you use can be raised up to prevent any or simply delete all temp files upon closing the browser up.
That will also mean re-entering all email and password information each time you go to log into any regular site you frequent like SF? As far as Google... Bing is a safer place!
Less bots and other garbage to worry about!
Different malwares apart from data miners tend to have other purposes like keyloggers, trashing the OS making it unusable, I-Worms replicating themselves using your email contacts to forward copies of the virus, and even fake wares to watch out for as far as getting you to believe you have a bug to con you into paying for a program that doesn't exist!
A good firewall will help while it's never 100% infalliable. Some of these things get on simply by user interaction where you get duped into something which then places things you don't want on. An effective web filtering like the free Web Of Trust
addon will tag questionable sites for you. If you register that will even offer parental controls for safe browsing.