You may need to go into the BIOS first and chang the boot order so as to have it boot to the cd first ,just look for boot sequence and set it to CDROM,A:\ ,C:\.
then just boot with the cd in the cdrom ,and hit R for repair.

Boot the system from the Windows XP CD. You may need to change the boot order in the system BIOS. Check the system documentation for steps to access the BIOS and change the boot order.

When you see the "Welcome To Setup" screen, you will see the options below
This portion of the Setup program prepares Microsoft Windows XP to run on your computer:

To setup Windows XP now, press ENTER.

To repair a Windows XP installation using Recovery Console, press R.

To quit Setup without installing Windows XP, press F3.

Press Enter to start the Windows Setup.

Accept the License Agreement and Windows will search for existing Windows installations.

Select the XP installation you want to repair from the list. This will usually be C:\Windows

Press R to start the repair.

Setup will copy the necessary files to the hard drive and reboot. Do not press any key to boot from CD when the message appears. Setup will continue as if it were performing a clean install, but your applications and settings will remain intact.
Review the Regional Settings and click Next

What is the BIOS and how do I go into that?

bios sometime called setup,on the first boot screen you should see hit DEL or f2 or f1 to enter ,bios /setup ,but it is not a place for a novice ,so be careful !