It is possible that at some point during using your computer that Windows Vista will be unable to boot. Corruption, viruses and failing hard drives can all cause this, but fortunately Windows Vista can auto-diagnose and fix most problems quite easily.
First, insert the Vista DVD into the computer and restart it (to start the boot process). You will be asked to press any key to boot from the DVD:
Then, fill in the language and location options and click Next:
Now, click Repair your Computer:
Then, select your Vista partition and click Next: