C:\Documents and Settings\YOU\Local Settings\Temporary Internet Files.
This folder needs to be emptied regularly for IE to keep working properly. When I used IE I did it about once a month. Nothing bad will happen. It just starts to fill up again & you will have to log onto some sites again.
Try going to Internet Options - Programs Tab & Click on "Reset Web Settings". This may or may not help - it cannot hurt. You can then go to the ADVANCED tab and put a tick in "Empty Temporary Internet Files Folder When Browser Is Closed."