First post i know but this may help you.
So you have the oem vd/dvd already which is part of your other system?
If so open the disk and look for the folder $OEM$
If its there then what you need to do is make a new disk without the oem part of it and boot from that, then it should work for you and you will put your own cd key into it.
Im pretty sure that will work as i had to do it for vista as my laptop hdd died and didnt have a backup dvd so used a friends vista. Put my own key in and all worked fine.
Btw... Just had a look on a dell xp cd i have and it has the oem folder so it should work.
If it dont help let me know and i will find another way.
Hope this helps