There are a lot of great advantages to using WordPress, but if you get to a point where you are seeing an Internal Server Error 500 every time you want to upgrade a widget, install a plugin or do anything useful, then it can be a real pain. Thankful I have a fix for you, especially if you are in a shared hosting environment like 1and1 or Godaddy.
Simply copy and past the following into your .htaccess file in the root folder of your blog and all will be well.
# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# END WordPress
Options All -Indexes
AddType x-mapp-php5 .php
AddHandler x-mapp-php5 .php