There are a couple of things you can try:
A. Boot into safe mode.
-Shut down your computer completely.
-Turn the power back on.
-After the BIOS splash screen (where you are told you can hit F12 to enter setup), hit the F8 key repeatedly.
-You will see a black screen with white text that is asking you how to boot windows. use the arrow keys to highlight SAFE MODE. hit enter to select it.
-You will see a ton of white text scroll down the screen with a huge list of file paths. This is normal. It may take longer to boot into safe mode than into normal windows. This is also normal.
-When you get into safe mode everything will look weird. The resolution will be set to minimum and "SAFE MODE" will show in all four corners of the desk top. this is normal.
-Back up the files you need and proceed with windows installation by booting from the CD. You may have to adjust the boot sequence in the BIOS. Consult your computer manufacturer's manual, or your motherboard manufacturer's manual for instructions on how to do this. You could also try to initiate the install in safe mode, by I have never had a need to try this so I'm not positive it will work.
B. Use Knoppix to bypass Windows security. Use this if you are not able to boot into safe mode.
-Go to http://www.knoppix.org/
-Click downloads. At the bottom of the page you will see download locations within the United States. Use those as you will get better speeds.
-Use a CD burning program to burn the disc image you just downloaded. You can do a search for Nero and download a trial version for free if you dont already have a burning program.
-insert the Knoppix CD you just burned into the bad computer.
-Again, you may have to adjust the BIOS boot sequence to boot from the CD.
-Your computer will display strange new things... from a strange new operating system. Knoppix loads directly from the CD into the RAM and does not damage your hard drive. Since knoppix is your operating system, you will no longer be required to enter in any windows passwords. You will have free reign to backup what you want. USB thumb drives and hard drives will work in Knoppix.
-A VERY nice feature of knoppix is that every time you reboot... you get a brand new, freshly installed, operating system. This is a great way to play with Linux without commiting to anything. If you destroy somthing... reboot and your back to normal :).
-Once finished, boot to the windows CD and reinstall windows.