Scripting errors can be difficult to debug. Have you tried flushing your browser cache? It's located under Tools Internet Options. Look on the General Tab, Browsing History.
Try to start IE (assuming that's what you're using) in safe-mode (no add-ons). It's usually located under All Programs, Accessories. If the error message doesn't appear, then you've probably got a conflict with one of your add-ons.
Check your Java runtime libraries too – visit Java.Com and install the most recent version for your OS.