You have a couple of methods........first figure out what the programs are that actually open the documents you want to back up because without the proper programs associated with them, the documents probably won't be viewable anyhow even if you saved them.
I would make sure then that you have the installation disks along with username/password/registration information for all of them and then back up the documents that go with each program. Put all those backups in a safe place along with the install disks that go with them.
If somebody else needs to view those documents, they would have to install the programs ahead of time and then restore/view the documents from the backup cds...assuming it would be done on another computer.
Second option.......store the entire computer in a safe location and just leave it be until it's needed.
Normally, I would say remove the harddrive and store it instead, but because it's XP, it is already geared up for that specific computer and if it's removed and taken to another computer, it will attempt to install all the hardware for the new computer and if it runs into problems, it may not be bootable. If the harddrive is connected as a slave, the documents will all be there, and if the master drive is able to read an NTFS harddrive, the programs associated with the programs may not run in order to view the documents on the slave since they are already installed on the slave drive and not the master so the registry may not be able to locate the program itself.
If somebody tries to hook up an XP slave to a FAT32 (W98 or ME) system, the files on the XP slave harddrive won't be seen, so that's not an option...just an observation. If the XP slave has been set up as FAT32 instead, then the W98/ME system would be able to see the files on that drive, but again, the programs associated with the files probably would have to be reinstalled on the W98 harddrive.
The first two suggestions I gave seem about the best options........hopefully somebody else will have more to offer.