I recently tried to install Drupal 6.2 on a freshlly installed Centos5 server (apache 2.2.3-11.el5_1, php 5.1.6-15.el5, mysql 5.0.22-2.2.el5_1.1 ). After creating the database tables the install script returned a page stating:

The installation has encountered an error.
Please continue to the error page.
An HTTP error 501 occurred.

After some research I found out that the above error was caused by mod_security apache module that was enabled. Editting

/etc/httpd/conf.d/modsecurity_crs_10_config.conf

and setting

SecRuleEngine DetectionOnly

solved the problem. This way I disabled modsecurity to detection only. Maybe there is a better way to handle this problem by leaving modsecurity on and performing per site or per directory configuration.