Which version did you install? Is your server configured for "Fast-CGI"?
If so, I think I can copy and paste the answer I just typed for another topic.
And I quote...
"I'd suggest that you download 2.0.6 RC1 and see if your problem goes away:
The download is here:
Your issue is related to Ticket 3215, which I've described in my 2.0.6 Feature List ( http://www.thecodecave.com/article297 ) :
"Ticket #3215 - "Login & Update functions everything is broken" - PHP can talk to the server in different languages. On servers using the “Fast CGI” language, an error 500 was received when reaching administration pages. The code was rewritten to have special handling for Fast CGI servers."