I have seen this problem all over the place on all sorts of forums but nobody has seemed to find any one solution to it, not to say that it's the same problem in every case. I can tell you however that I had the same problem on my Dell Inspiron 1520. At first it seemd that it was a random occurance, sometimes the computer would boot up, other times it would get a black screen with just the cursor visible. I searched and could not find a soution that worked for me, eventually I found that the problem while still not happening all the time would only happen if I had a network cable plugged in or a wireless device trying to active connect when booting up the machine. If my machine booted to a blackscreen I could spend all day turning it off an back on with no luck however if I turned off my wireless and unplugged my network and then restarted the computer it always came up in one try.
Don't know if that will help but you can also try searching google for something like "Vista boots to black screen" and you'll see that there are tonnes of issues with this.