I have done everything the install guide states, but I still get the following error when accessing the installer page (http://localhost/dotnetnuke/install/install.aspx?mode=install):

DotNetNuke Configuration Error

Object reference not set to an instance of an object.

DotNetNuke has extensive file upload capabilities for content, modules, and skins. These features require custom security settings so that the application is able to create and remove files in your website.

Using Windows Explorer, browse to the root folder of the website ( C:\DotNetNuke by default ). Right-click the folder and select Sharing and Security from the popup menu ( please note that if you are using Windows XP you may need to Enable Simple File Sharing before these options are displayed ). Select the Security tab. Add the appropriate User Account and set the Permissions.

If using Windows 2000 - IIS5
- the {Server}\ASPNET User Account must have Read, Write, and Change Control of the virtual root of your website

If using Windows 2003 - IIS6
- the NT AUTHORITY\NETWORK SERVICE User Account must have Read, Write, and Change Control of the virtual root of your website

I have even set the filesystem security to full control for everyone on the dotnetnuke folder, subfolder and files, but it does not make a difference.

I am running W2K3, IIS 6.0, SQL2005, ASP2

Thank for you advice.

My NETWORK SERVICE is FULL control permission, NTFS permission is OK read/write enabled.  I not use Front page, my database name DNN431, Virtual directory is DNN. I refresh browser and delete cookies.  If I go in Webserver ASP.NET setting I see required permission denied and not open this window.

double check if you haven't already - especially for the portals folder and web.config file.  Make sure SERVERNAME\NETWORK SERVICE has FULL permissions for W2k3 and the same for ASP.NET account for WinXP Pro. 

The error indicates it can't write to the files.  Make sure the NTFS permissions is the last thing you do/check - since unzipping or setting IIS settings for FP can screw this up.  I spent many hours on this error when starting out. 

for your OS Win2k3 - only the NETWORK SERVICE needs these rights - as long as you have not done any changes in the AppPool.  Open IIS - > expand Application Pools, then right click on your default AppPool.  Go to the far right tab (Identity I think).  Make sure it shows NETWORK SERVICE account.  If this is something else, then that's the account to give full permissions to on the DNN folders.