I'm sorry I forgot a step!!!
1- Copy data.dat from \Documents and Settings\All Users\Application Data\Microsoft\Office\Data and save it to a partition you will not be formating or a flash drive or floppy or CD.
2- Open a command prompt and type vol c:. Replace C with the appropriate drive letter if you have XP on a different drive.
3- Copy the 8 characters (two groups of 4 characters separated by a hyphen) you get from vol c: to the same place you saved data.dat.
4- Re-install XP.
5- Activate XP.
6- Download the volumeid utility.
http://www.microsoft.com/technet/sysinternals/FileAndDisk/VolumeId.mspx
7- Run volumeid and change the volume id of the XP partition to what it was before format (you saved this in step 3).
8- Reboot. Open a command prompt and type vol c: (replace c with the drive letter of XP) to confirm that the volume id is changed to what it was before format.
9- Install Office.
10- Copy data.dat to \Documents and Settings\All Users\Application Data\Microsoft\Office\Data. Done!
The volume id is one of the parameters Office monitors to decide if too many components have changed and a re-activation is needed.
By setting it back to what it was, you reduce the number of changes, which otherwise could trigger a re-activation.