Internet Explorer 8 is a new release and some websites may not yet be ready for Internet Explorer 8. Click the Compatibility View toolbar button to display the website as viewed in Internet Explorer 7, which will correct display problems like misaligned text, images, or text boxes. This option is on a per site basis and all other sites will continue to display with Internet Explorer 8 functionality........
The Microsoft page linked to above has pictures on how to switch to Compatibility View and how to maintain a list of sites that work better in IE7.
As a rule of thumb, it is sometimes best to do research before installing. Sometimes installing something because it's the newest version is not always the best thing. Many times there are kinks that still need to be ironed out.