I was trying to install SharePoint Server 2007 on a Windows Server 2008 R2 with SP1, but every time I run the SharePoint Products and Technologies Configuration Wizard, I bumped into:

Performing configuration task 1 of 4
Initializing SharePoint Products and Technologies upgrade…

Successfully initialized SharePoint Products and Technologies upgrade.

Performing configuration task 2 of 4
Initiating the upgrade sequence…

Failed to initiate the upgrade sequence.

An exception of type Microsoft.SharePoint.Upgrade.SPUpgradeException was thrown.  Additional exception information: One or more types failed to load. Please refer to the upgrade log for more details.

If you look into the Upgrade.log file you will see the following associated exception text:

[SPManager] [INFO] [7/19/2011 2:21:16 AM]: Upgrade Boot Strap starts. Initialized in Mode: InPlace, as MachineAdmin: True.
[SPManager] [ERROR] [7/19/2011 2:21:16 AM]: No connection string to a WSS configuration database detected.
[SPManager] [ERROR] [7/19/2011 2:21:16 AM]: SPManager.Initialize failed.
[SPManager] [ERROR] [7/19/2011 2:21:16 AM]: No connection string to a WSS configuration database detected.
[SPManager] [ERROR] [7/19/2011 2:21:16 AM]:    at Microsoft.SharePoint.Upgrade.SPManager.Initialize()
[SPManager] [INFO] [7/19/2011 2:21:16 AM]: Upgrade Boot Strap finishes. 2 errors and 0 warnings encountered.
[SPManager] [ERROR] [7/19/2011 2:21:16 AM]: Upgrade failed.
[SPManager] [ERROR] [7/19/2011 2:21:16 AM]: No connection string to a WSS configuration database detected.
[SPManager] [ERROR] [7/19/2011 2:21:16 AM]:    at Microsoft.SharePoint.Upgrade.SPManager.Initialize()
at Microsoft.SharePoint.StsAdmin.SPUpgrade.RunUpgrade(StringDictionary keyValues)

How to solve

1. Open command Prompt – Type ‘regedit’,
2. Navigate to the  HKLM\SOFTWARE\Microsoft\Shared Tools\Web Server Extensions\12.0\Secure\ConfigDB

Check  for the  dsn key for connection string.  If dsn doesn’t exist, then add it manually as a work around and  try to re-run the SharePoint Products and Technologies Configuration Wizard.

• Add a new string key named dsn with the value Data Source=<db server name>;Initial Catalog=<config db name>;Integrated Security=;Enlist=False