The Windows Recovery console allows you to boot into a CLI which you can use to resolve issues when Windows no longer boots. Normally, you would start the console from the installation CD/DVD, but you can also install it so the installation media is no longer required. Instead, you get an additional option in the boot menu where you can select it.
To install it, insert the installation media, open up a command prompt and run these:
x: cd \i386 winnt32.exe /cmdcons
This will install the recovery console. If you get an error about a too new version, you may have to slipstream the latest service pack into your installation media. You can also run the installer from a local path (i.e. copy to network and run via UNC, or copy it locally to HD, and then run the installer).