IIRC, the OS supplied with Dell machines is specific to Dell and actually looks to see that is what the hardware is. So, could it be that your XP disk is not the Dell variety and is being rejected?
I believe that a Dell-branded CD / DVD will only work with a Dell PC. However, a genuine retail Windows CD / DVD will work on any platform. So that is unlikely to be the problem here.
You can use the Win XP disk to boot and then erase all existing partitions and then re-partition as required, prior to carrying out a clean install of XP. That would be the best way to get a clean install of XP.
However, if Dell has not released any XP drivers for that model, you may find that the system (or certain components) won't function.
If the system does work, but you are missing component drivers, you may find equivalent drivers direct from the manufacturer - provided you can identify the underlying hardware.
SD