http://support.microsoft.com/default...;EN-US;q260650
if you dont feel like clicking here is a rundown on what it says.
CAUSE
There are several reasons why this issue may occur.
This issue may occur if your Temporary Internet pages cache is full.
This issue can occur if you have "Do not save encrypted pages to disk" selected in Tools, Options, Advanced Tab and are loading the page over a secure connection (https).
This issue can occur when you connect to a Web site using an address that contains < username:password>. For example, the server is accessed through
http://< username:password@site>/default.htm, instead of
http://< site>/default.htm.
there is also a post at
http://www.annoyances.org/exec/forum/win2000/n998605951 which asks the same question. I haven't read all the replies but its possible that there is another fix there. I had this happen to me 2-3 days ago and clearing cache and restarting fixed it.
SM70 Edit: post fixed so that smilies don't replace parts of the example URL