After upgrading my Windows XP to service pack 3 (SP3), IE7 was acting really weird. Most pages didn't show up correctly in IE7, and the Alexa toolbar plugin was completely hosed, it just showed a big white bar where it normally sat. I eventually figured out that most of the websites where messed up because none of the Javascript was firing. I looked at all my settings and it should have been working, but none of it was, and there were no errors.

I'm not really sure why this happened, but the fix was to reinstall IE7 after upgrading to XP SP3. After I did that, javascript started working again as did the Alexa toolbar.