It sounds likes your scren refresh rates are out, or maybe the monitor is beginning to give out.
To get to safe mode, you need to press F8 repeatly after the Bios POST, but before Windows Vista starts to boot until you get to the Vista boot menu.
From the menu you can select various options, of which one is Safe Mode, and another would be Low Resolution Mode (I'm not sure of the exasct wording on that one, as I haven't had to use it for some time).
If you can get to Low Res Mode, then your display driver is corrupted and needs to be re-installed. If not, then try getting into Safe Mode and rolling back your driver.
If neither works, then its possible that your monitor is beginning to fail and needs to be replaced. I've just a couple of days ago had a case like this, where the PC begins to boot, Windows starts, but as soon as XP got to the login screen, it went black. Replacing the monitor resolved the issue. Using the "faulty" monitor on a different PC produced ther same result, confirming that the monitor was giving out.
Hope this info helps.