If you use FireFox on any platform, update to version 188.8.131.52 to fix (more) critical, exploitable vulnerabilities. Here's the download page for Mac, Linux and Windows: http://www.mozilla.com/firefox/all.html
If you use Thunderbird, same story: http://www.mozilla.com/thunderbird/all.html
While you're at it, update your Flash Player to version 9, there was a massive exploit going down at MySpace recently that exploited vulnerabilities in Flash Player 8: http://www.adobe.com/go/getflashplayer
Also uninstall your older version of QuickTime and get the latest version, to fix security vulnerabilities in QT: http://www.apple.com/quicktime/download/standalone.html
If you use Windows, run your system through the Windows Update site a few times until it's got all the Critical Updates installed: http://windowsupdate.microsoft.com If you have any Microsoft Office software (Word, Excel, Outlook, PowerPoint, Publisher, Visio, etc) then make sure to hit Office Update a few times too: http://officeupdate.microsoft.com
If you're a Mac user, use the Apple Update feature to make sure you're all up-to-speed too.
Lastly, if you use OpenOffice, guess what? Yep, that needs some security vulnerabilities fixed too! Get the latest version from http://www.openoffice.org
Also, if you have no antivirus software, or it's some relic from Long Ago, there's fairly good antivirus that's free for home use, such as AntiVir: http://www.free-av.com Make sure to go through the configuration panels and fully configure all the goodies so it's not running in Nerf mode.
Stay safe out there