On a Windows system using IIS:
- Right-click on sites\default\settings.php and grant Write permissions to IUSR_MachineName (IIS6) or IUSR (IIS7).
Server 2008: You can also do this from the command line from within your sites\default directory:
icacls settings.php /grant IUSR:W
The installer will change the file back to Read Only after installation, but you should verify this after installation.
For more information about modifying Windows file permissions, see the Troubleshooting FAQ.