SQL 2005 Upgrade Gotcha
Damn, this sorta thing makes me so mad....!
I've just been fighting with a SQL Server 2005 Enterprise Edition upgrade at my employer; it should have been a simple upgrade from SQL Server 2000, but halfway through the installation process I was getting a "SQL Server setup could not validate the service accounts" error. Following the online help led me to a help page that suggested the problem was to do with password strength. Cue several hours following a red herring.
After examining the Security log, I had a light-bulb moment; the old SQL instance had the service accounts in the user@domain format. Changing the service account to the older DOMAIN\user format in the Services control panel and restarting the installation process allowed the upgrade to happen. Easy once you know how.