For personal use, I usually get the OEM copy.
In your specific situation, what you need is a license.
One thing to try, is to determine the history of this PC, and see if it should have a valid license.
Do you have the original Certificate of Authenticity for XP on this PC?
If so, you might be able to call Microsoft and get an updated Product ID that will take care of things.
If you don't have the CoA, and cannot determine if the copy of XP on the PC is legitimate, then you need either a license, or a new copy of XP.
Do you have the XP CD?
The OEM copy has a statement on it to the effect of:
"For distribution only with a new PC."
The upgrade version should have the word "Upgrade" on the CD.
If you have just a restore disk, and the system is a name brand, try contacting the original manufacture. They might provide a new key to you.
Keep in mind that the upgrade version requires that you have either Windows 98, Me, or 2000 to qualify.
You will need the original old version CD should you ever need to re-install XP clean.
Chas