There seems to be an increase in the "I can't install DNN using the web platform installer" posts on the Dotnetnuke forums so I thought I'd make a post about it.
From what I can see the issue revolves around the sa account in SQL Server 2008 Express. The installer is rejecting the password but this is normal since it wasn't prepped correctly. Most people are getting SQL Server 2008 Express installed as a pre-req to installing DNN and it needs a few tweaks:
1) Log in to SQL Server 2008 Express using .\sqlexpress and windows authentication
2) Expand Security, then Logins
3) Look at the icon. If there's a small arrow pointing down, the account is disabled. Let's fix that.
4) Double-click the sa account to open properties.
5) Change the password and confirm it.
6) Click on "Status" in the left pane to reload the options in the right pane. Select "Enabled" under the header "Login".
7) Click "OK" to close the properties window.
With the sa account taken care of, we need to check to ensure we're in mixed mode:
1) While in SSMS, right-click the server name in the left pane and select "Properties"
2) Click "Security" in the left pane and select "SQL Server and windows authentication mode".
3) Click "OK".
4) Close SSMS.
The last step is to restart SQL Server (SQLEXPRESS) under control panel --> administrative tools --> services.
Once this is done, log in to SQL Server 20008 Express using the sa account and password that you changed. If successful, you can now install DNN using the web platform installer.