I would try using an old boot disk from
http://www.bootdisk.com/ and re-formatting your hard drive.
You can get the XP disks from here too.
If you have set the boot list to D,C, A etc and the CD is not booting I presume there is a problem with either your CD Drive or the XP CD...........is it genuine or a copy?
If you are having connection problems I would unplug a the cables and re-plug them all in again just to double check.
If this fails you would have a hardware problem either Hard Drive, CD Rom or Mobo, finding out which is a process of elimination disconnect each and test, and try different components if possible.