So far I’ve seen error 500 pop up only in two occasions:
Sending a wrong header with the php function header(), and a badly formed .htaccess file.
Drupal installs a .htaccess file in its directory automatically, so perhaps something went wrong with it. I’d rename it to something like 1.htaccess (thus deactivating it) and see what happens.
If the error goes away after that, you should post the content of the .htaccess file here.
What version of Drupal did you install - 4.7.3?
Edit: I just checked the page and it said (of course) access denied. But that means the Drupal engine actually gets to run, so it can’t be an .htaccess error or something like that…