I've been having the same problem the past couple of days on a Toshiba laptop, it just won't boot at all into Vista. Most of the time the screen is black and has the Microsoft bar with the green lines scrolling across it.
I have managed to get it to start in safe mode a couple of times, but usually it just keeps trying to restart or hangs.
When in safe mode I found a couple of updates that were installed not long before it started playing up and I uninstalled them. It hasn't made any difference.
I made a recovery disc thanks to the tutorial on this great site but it wasn't able to repair and system restore won't work either, I have tried going back to a few different points prior to the updates.
A couple of error messages appeared simultaneously when it appeared as tho it may boot up to the desktop normally, these were:
"Explorer.exe - Application error"
The instruction at 076914ce6 referenced memory at 0xfffffffe4, the memory could not be written. Click ok to terminate.
"Location is not available"
C:\windows\system32\config\systemprofile\desktop refers to a location that is unavailable. It could be on a hard drive on this computer or on a network. Check to make sure the disk is properly inserted, or that you are connected to the internet or your network, and then try again. If it still cannot be located, the information might have been moved to a different location.
Sorry this is so long but I understand that the more information that is available the more likely that someone may have a clue as to how to resolve it.
I thank you all for your time and would be most grateful if anyone can offer some advice.