It's quite possible your PC still has the factory restore partition - in which case, you do not need to 'find' an ISO online - you won't even need the key, actually.
Assuming you've backed up everything you want to keep, restart the PC and hold Ctrl+F11 as it reboots.
If you see the "Dell PC Restore By Symantec", you should be good to go, as this holds a disk image of the initial (i.e. factory) XP installation.
Select that option, and let it work its magic - this is fully automated, and doesn't require any input. (it may take a bit of time, but it is nevertheless faster than a manual re-installation).
- If the disk was repartitioned at some point in its life, the restore partition may be gone.
- This is different from a 'recovery CD'.
- Once the fresh disk image is restored, you will have to follow through with a (long) session of drivers and Windows updates, as the OS reverts to the state it was whenever that computer shipped. Not a big price to pay for a nice clean install though.
- Any third-party software that wasn't part of the initial factory setup will have to be re-installed, obviously.
- It remains up to you to figure out if you can legally use this OEM license (possibly a Volume License, if you've got this out of a company pool) - See Billy_Bob comment to your initial question.