My computer was running with W7 RC1 very well for a while, but since I had to reboot because of performance problems, it doesn't boot anymore. It goes through the loading screen, but after it keeps trying to open a user session ("Welcome...." with the blue wallpaper).
So I tried to create a new user session in safe mode, but now I have a black screen with the cursor (instead of the user selection screen).
If Safe Mode doesn't even work, I would stick the W7 RC disk in the DVD drive and boot from ithe drive. There you should see the option to repair start up errors. Some files for initiating Windows are perhaps messed up. If that doesn't work out, you can always back up your data files by connecting the HDD to another computer and extracting them to another HDD, and then clean insall the OS. But I highly doubt you have to go that far to fix this (I hope not).