Can I use a windows XP disk that came with my dell laptop with my HP desktop computer?

No. The disk that came with the DELL laptop does not contain all the files that a full Windows XP installation disk comes with. Instead it will contain the files needed to install Windows XP on your DELL laptop. As Windows XP installs it will detect the hardware that your computer is constructed from (including the motherboard and processor) and install files that are specifically for that hardware. Your HP computer will have different hardware to the DELL, so the installation that is suitable for the DELL will not work properly on the HP.