There's a nice tool that can save your a** from time to time and it's call Recovery Console:
To install the Recovery Console, perform the following steps:
1. Insert the Windows XP CD into the CD-ROM drive.
2. Click Start , and then click Run .
3. In the Open box, type d :\i386\winnt32.exe /cmdcons where d is the drive letter for the CD-ROM drive.
4. A Windows Setup Dialog Box appears, which describes the Recovery Console option. The system prompts you to confirm installation. Click Yes to start the installation procedure.
5. Restart the computer. The next time you start your computer, you will see a "Microsoft Windows Recovery Console" entry on the boot menu.
NOTE : Alternatively, you can use a UNC to install the Recovery Console from a network share point.
Hope this help