It probably has something to do with a 32bit library vs. a 64bit library. 'procexp' might show you that.

Curious, why are you going to SQL Server 2005? You're losing support in April, my opinion would be to upgrade to one of the 2008 flavors (if that's an option).

Another shot in the dark...when you set the owner of the SQL Server Agent service, did you use the SQL Server Configuration manager? When done thru that interface, it creates the necessary Windows groups and add those groups to the necessary Local Security Policy's. Since you said you modified the registry, I'm assuming you followed something like:

http://support.microsoft.com/kb/283811

Maybe double check that setup?