Monitoring can be done with softwares. Some firewalls offer
it... but they get very irritating, especially if you are
installing software. Every little thing, you get prompted.
There are others as well... like Winpatrol, System Safety
Monitor, ProcessGuard, etc.
Viruses strike computers in a lot of ways, but most notably
would be through loopholes of a program or the operating system
itself. Windows patches itself automatically on the 2nd Tuesday of
each month. If there are critical bugs, there will be more
patches.
Softwares wise... it's up to developers to patch it. Some
patches more often, like browsers. Others not so.
Nowadays... viruses no longer just spread through loopholes
found. Embedded files like Java applets, Flash files can be a
cause. More and more are also using JavaScripts to spread
viruses... for Internet Explorer, not only JavaScripts can be used,
ActiveX are used as well. It's the prompt you receive from IE if
you want to install something...
Advertisements in lots of websites are also being used to
spread...
Just because you practise safe surfing habits doesn't mean you
won't be hit. One JavaScript alone is enough to wreck havoc...