If you use Flash Player (which is probably 96% of us) update it to the latest version. I think this link will work for all OSes, let me know...
Adobe says this update fixes a dozen security vulnerabilities. If you have both Internet Explorer and other browsers, update both. Chrome updates its own Flash Player, so if you're using Chrome, it should take care of this for you.
You can check your browser(s) for a variety of out-of-date extensions/add-ons at Mozilla's no-brainer checkup page, too. http://www.mozilla.org/plugincheck/
Bah, my OS is too l33t for that security stuff, since I don't use M$ Windoze! Update your stuff anyway I recall a Flash exploit that worked on Mac, Linux and Windows regardless of the web browser, and it wasn't just a proof-of-concept either... it was being used in the wild. Always best to keep your browser-related stuffs up-to-date