I ran a registry cleaning and when I turned my Dell (windows 7) on here is what happens:
the black screen with one cursor line in the upper left hand appears, as is normal. However, it takes at least half an hour to move to the next screen.
The next screen shows the progress bar, but it moves extremely slow. This is the same screen which shows the two options from F2 and F12 (setup and boot options).
it takes at least a half hour to advance to the next screen.
The next screen is the "Windows is starting" screen with the Windows logo. I let it run for over twelve hours like this... it eventually just was a black screen and the hard drive eventually went into hibernation mode and wouldn't do anything else.
You can actually repair a lot of windows problems with a Windows install CD. In this instance you'd boot into the Windows Install CD, select Repair My Computer (assuming one of the automatic repairs doesnt fix the issue,) select command prompt and you can run a few commands that might help you boot,