Hi Karode,
For as many times as I have installed DotNetNuke (...it's always fun to see that "Click here to access your portal"), everyonce in awhile it seems like I hit every possible combinations of incorrect entries that would prevent the install from working.
Anyway, I believe you have covered all the basis and can get your site working soon. Below are some thoughts and items I would recommend, some (...but hopefully not all 
) of which you may have tried:
- Delete your DNN database if you believe the install script, which kicks off the first time you navigate to you new site (sounds like this may not apply here).
- With all that you covered in your post, my next recommendation would be to open IIS 6, drill down to your DNN website root directory and locate a static htm file, if exists, or a gif file, i.e. 403-3.gif.
- Right click the file of choice and click "Browse". This will (or should) cause your default browser to lauch and show this file in your browser. Note the URL to confirm where IIS thinks your root directory is, i.e. http://localhost/DotNetNuke/403-3.gif for example. Modify any IIS 6 settings, if necessary, but most likely this will be as you expected.
- Also check and note your port number if you have changed the value from the default port 80.
- In IIS6, check that your website has a Default.aspx page in the Documents list tab (right click on your website, click properties, and click the “Documents” tab. Add the default.aspx value to the list if you do not have such an entry.
- Next, get the machine name of your Win2k3 server and keep handy – you probably have this but if not, you can right click “My Computer” on your desktop, and click the “Computer Name” tab to locate this.
- Open SQL 2005, SSMS, and if you did not delete your database, as suggested in step1, see if you have the PortalAlias table.
- If you do check the values in this table and see that the HTTPAlias value is “MachineName/DotNetNuke:PortNumber (Of course, if this is an Internet facing website you will want to change this value to “www .yourwebsite .com”, but this change can be made after you are done setting the site up on your server. For now you can enter http://MachineName/DotNetNuke:PortNumber, for example.
- If you do not have any user tables created by the DNN installation, make sure that you make any necessary changes in IIS6, and try a refresh of the DNN website files by deleting your current DNN website files (keep your web.config for your SQL Connect String, if necessary), delete and re-create your DNN database with the consistent DB name, uid, and pwd used in your site’s web.config file, and add /check the Network Service uid providing it with the proper NTFS permissions just as you mentioned in your post.
- Open IIS6 MMC and again drill down to your DNN website’s root folder. Locate the default.aspx page, right click it and select “Browse”. Note any inconsistencies in the URL from what you expected (refer to step 3).
As will all things this still may not get you beyond your issues, but hopeful helped eliminate potential setup issues. If you do not receive feedback from DotNetNuke’s installation page, you may want to check your Event Log’s Application log to see if there is additional information here. IIS6 out of the box (setup w/o modifications) is locked down, as you may have noticed, and it there may be a need to permit aspx and other resources to run.
I mentioned some very trivial steps and this was in no way meant to question your technical skills. I simply don’t know your skills but have a feeling you have done or checked most, if not all, these steps. If so hopefully you can get at more information to determine what is failing and perhaps others who have not been down this path with Win2K3/IIS6 can learn as well.
Best of luck and post back if you are still stuck.