You need to set the CD-ROM as first boot device in the BIOS.
Then, as the computer boots you need to press a key to boot from the CD-ROM
when prompted by a message on the screen . You have a limited amount of time
to do this.
Windows will load setup files and, if your instructions are accurate, you
will need to proceed a few steps into the setup procedure at which point you
will receive a message that "an existing installation of windows has been
found, do you wish to repair windows", or something to that effect. If I
understand your intent you probably don't want the "Repair Console", so
don't choose that option when offered.
This is assuming the current installation is not corrupted beyond
recognition.
After you finish the repair you may need to reinstall service pack 2, and
will probably need to install a few updates.
Having said that, not all CD-ROM drives will boot, (older ones usually), but
most do. If you have two CD/DVD drives put the disk in the drive set as
master if they are connected via IDE cable.