If you run into a situation where Windows Vista won't boot at all (not even to the point where you get the menu where you can choose to boot Safe Mode etc.), here are a couple of things to try:

First, boot off the DVD, and select Repair. Assuming it finds your installation of Vista, it has an option to automatically repair startup problems (if it can't find your installation, you may need to load a driver for your IDE/SCSI/RAID controller from a CD; this is fairly straightforward in Vista, unlike previous versions of Windows).

If it says it can't find any problems, try rebooting the computer with the Vista DVD in the drive. When it says “Press any key to boot from CD or DVD.....” just wait, don't press anything. If Windows boots when you do this, but doesn't boot without the DVD, the problem isn't Windows, it's your BIOS! I solved this by removing the battery for a minute, but there was an option in the BIOS setup to reset the configuration; I'm sure that would have fixed the problem just as well.

If, on the other hand, Windows still doesn't boot with the DVD in the drive, and the automatic repair option didn't fix the problem, try this: boot from an old Windows 98 CD, get to a DOS prompt, and type “fdisk /mbr”. This will overwrite your MBR and make Windows unbootable, but now Vista's repair tool should see this problem and will try to fix it. In doing so, it may solve your other problem at the same time.

Hope this helps!