This will not help in your case, but I had a similar problem and googled to this place.

I have had this problem with IE6, I got the error message;

"Unable to initialize the viewer. If you have installed the viewer to your computer, please re-run the Setup program. If you are accessing the viewer from a server, please contact your System Administrator"

I found this link;

Explains if SQL2005 is installed you will get this error.

I do not have SQL2005 server installed, but I do have enterprise manager 2005. That is probably enough to stuff it up. It is caused by a conflict with msmxml6.dll.

Solution: unregister that dll. Note this will probably break SQL so you might want to just use a different PC to access MELL.

Edit: this might work http://support.microsoft.com/kb/917583