The System File Checker (sfc.exe) is a command-line tool built into Windows that helps you repair corrupted system files — but what if you can't boot your PC? The Winhelponline blog explains how to use it from the install disc.
Normally you'd just open up an administrator mode command prompt, and then type in
sfc /scannow to scan your system files and either repair or replace anything that's broken — but when you're booting off an install DVD it's not quite as simple. Instead you'll need to boot off the install disc, choose the "Repair your computer" option, choose your installation drive, open up a command prompt and then type in this command:
sfc /scannow /offbootdir=d: /offwindir=d:windows
You'll need to adjust the drive letter depending on where your system is installed — you'll probably notice that the drive letter is different during the repair process than normal, so make sure to pay attention on the screen where you choose the installation to repair.