|DOS has limits on the amount of mamory it can access. Basically the first 640KB of RAM unless more has been configured. |
Instead of trying to re-invent the wheel I suggest you figure out what is going on with the laptop.
Seeing as there is no DOS on the WinXP CD, unless you have cobbled the disk structure, I think you may be reading something off the hard drive.
What is/was on the hard drive?
When you attempt to boot from the WinXP CD do you see a prompt to hit any key to boot from the CD? If you don't then either the boot order is wrong or there is a problem with the WinXP CD. Because you are able to boot to a different CD then the issue points to your WinXP CD as the problem.
Try the WinXP CD in a different computer to see if you can boot to it then.