This is a problem with your browser, not the forums.
Are you using IE7 or IE8? Have you run Windows Update recently?
I would recommend doing a malware scan using Spybot or MalwareBytes AntiMalware. If you don't think you're infected with anything, then try these troubleshooting steps from Microsoft:
Disable Add-ons. If the problem goes away, one of your add-ons is causing the problem:http://blogs.msdn.com/ie/archive/2006/07/25/678113.aspx
Reset your IE7 settings. This is kinda extreme as it wipes all your settings and plugins and stuff:http://support.microsoft.com/kb/923737