Vista slow bootup after password removal

I was running Vista Ultimate x64 on my new system and bootup was fine and zippy. I then added a password to my user account (the only account set up) but when I removed the password bootup was significantly slower. There is now a 25sec pause at the welcome screen followed by a black screen with just the cursor showing for another 30secs.

I tried using msconfig to prevent unneccessary items loading at startup but to no effect. (Apart from Vista blocking MSconfig from running at startup... sigh!) Selecting 'diagnostic startup' which loads only the bare minimum services and suddenly i don;t even see the welcome screen or black screen, I got straight from the Vista logo splash to desktop!

So, without rebooting a hundred times with another service enabled each time, does anyone know what what may be causing the slow boot times?