500 internal server error while installing
As Clive says above, it's nearly impossible to help without knowing more about your particular setup and debug info.

Common pitfalls are :


Various php.ini settings : max_execution_time as mentioned by monymirza above, but also memory_limit. This last can be raised during install and dev, then lowered for production.
Lacking .htaccess at Drupal's root. Standard unix commands often don't copy hidden files, and .htaccess is more or less mandatory for D7 to work.
Lousy DB connection, particularly on shared hosting.
Misconfigured vhost : perhaps a particular setting messes up things between domains, or subdomains. Also, you have to check the AllowOverride All directive on Drupal directory for it to function properly.


There are others, but these should get you started.