Right now I'm in the process of upgrading a Dell Latitude D820 to Windows Vista RC2 - currently running RC1. I installed RC1 Build 5600 as a clean install about 2 weeks ago.
Now I know that most self-respecting IT bods will never really do in-place upgrades .. I figured lets try the "what-if" factor.
When I tried to kick off the RC2 setup, it pops up an error message saying there's insufficient drive space on the computer and that I need ~400MB to start the install. HUH? I have 30GB+ available on both my C: and D: drives!
Wait, maybe because I'm running it as Domain Admin, not as the computer's Local Administrator. That's happened before when trying to install certain software and device drivers. Nope, still no go.
Let's try booting from the DVD. Nope, the UPGRADE option is disabled with a nice message saying, "Sorry dude, if you wanna upgrade, do it from within Windows!" GRRRRR!! 
Let's see if there's a "No Hard Drive Space Check" switch that I can use. SETUP /? doesn't bring up anything. I stopped looking - does anyone know if there's such a switch?
OK - maybe because I've got a virtual CD-drive installed as well, that could be causing it? Disable, try again. Insufficient drive space. What else? Check for any portable drives, USB sticks, is my camera still plugged in? Nope.
Drive Manager - What's this I see? The DELL Utility partition is the first partition on the drive .. and only 39MB in size. I'm guessing Setup is looking at this partition and then fails? Well, since I won't be able to upgrade and will probably have to do a clean build anyway, let mess around for a while. What if I set the C: partition as the active? It should be set already, but again "what-if"
NO WAY! IT WORKED!
There you are: Mark you primary partition as active (again?) and you should be good to go!
Makes you think: what would happen if a home user with Dell computer want to upgrade to Vista? Reckon this would be a problem? If anyone upgrades Windows XP to Vista RC2 on any PC with a utility partition, please let me know!
- Marlon