I've recently migrated two of my PCs to Vista. I was trying to wait until Windows 7, but I had to physically replace the boxes so my hand was forced into (yeah, I could have downgraded, but I figured I should actually work with Vista a bit before moving to Windows 7 if for no other reason that to appreciate it more.)
Anyway, one of the problems I've had was logging into the boxes using the administrative shares. Well this is disabled by default (and for good reason,) I needed a way to access via the shares and came across this article from Microsoft:
Error message when you try to access an administrative share on a Windows Vista-based computer from another Windows Vista-based computer that is a member of a workgroup: "Logon unsuccessful: Windows is unable to log you on"
This has allowed me to access my box from Windows XP and my other Vista boxes when I log in w/admin credentials—which is exactly what I wanted to do. While this isn't a recommended thing to enable, it seems you can toggle it off/on without rebooting—which is nice.