Just a quick update to a previous post on network drives slowing down the boot sequence of Windows XP. Turns out that I was too quick when I stated that Windows Vista doesn't seem to suffer from the same problem.
In the last few days I changed which shares were being mapped to drives and I suddenly started finding that logging in to Windows Vista was taking ages. It would just sit there with a black screen and mouse pointer apparently doing nothing. So I've just added the registry key from the previous post, rebooted and everything seems back to normal.
So to recap, don't rely on Microsoft to fix things and if you are getting a slow log in under Windows Vista and have mapped drives then it's worth trying this fix.