Vista won't let you start your computer
There can be different causes for a system crashing, but when you can not even start the computer in Safe Mode, you need a Vista Recovery disk.
First, how to make a recovery disk
How to create a Vista Recovery Disk for Hard Drive Recovery: by Dianna Monda Dill. If you can not start your computer using the normal methods, you will need a Vista Recovery disk to get into your system to work on problems. Here are instructions for creating a recovery disk. This utility is available on the Vista Business, Ultimate, and Enterprise editions.
Once you have your Vista Recovery Disk, Dianna's article How to Perform a Hard Drive Recovery Using Vista's Complete PC Restore will guide you through the process of recovering your hard drive and completely restoring your system from a crash. Again, this utility is only available on the Vista Business, Ultimate, and Enterprise editions.
Many of us do not have a Business, Ultimate, and Enterprise edition of Vista. Since Dianna's articles only deal with the high end editions of Vista, Lamar Stonecypher has written an article on ways to Create a Bootable DVD or CD in Vista for editions of Vista like Vista Home Basic, and Vista Home Premium, which do not come with the ability to create a recovery disc. He provides three methods of making a bootable disc which allow you to bootup Vista in the case of catastrophic hard disk failure. He also links to his article on various methods of backing up Vista so you can make sure you have copies of the material on the inaccessible hard drive before you can not boot up Vista or suffer catastrophic failure.
Four Boot Disks to Help you recover from System Failure by Peter N, has a section on a Vista Recovery disk. The contents of the disk were released with a beta version of Microsoft Vista SP1, but were not included in the final version distributed through Windows Updates. He gives you a link to a site which has a download of the Windows Recovery tool. You will need to burn the .iso file to a disk before you can use it to access the repair environment. It is not an installation disk.