Issue – Unable to open a website after upgrading to IE8.
Cause – This might be due to some incomptibilities and to isolate the root cause, do a trial and error check and identify if it is issue with IE8, or the website. You need not uninstall IE8 for this simple reason 
Resolution – Perform the following:
Close all the other browsers except the one in which you are trying to open the website.
Click Tools, Developer Tools (You can click F12 key instead)
Now, if you are able to open the website, it is due to the compability of some of the files such as javascript files, or ActiveX files etc with IE8. You may look at the Download section of IETips.net and download the re-registering .BAT file to re-register the files, which may resolve the issue.
Does Internet Explorer seem unstable to anyone else?