A lot of virus infections (most, actually) come from exploits in out-of-date browser plugins.  A common trick is for nefarious websites to popup messages saying (e.g.) "You must update your Adobe Flash!" and users end up installing toolbars and other garbage into their systems.  Don't fall for it.

I had been recommending a page from Mozilla but they've recently changed it so it only works with their Firefox browser so here's an alternative for other browsers.

Click then bookmark this page for future use:
http://www.check-and-secure.com/browsercheck/_en/