I assume you're upgrading to XP pro.
Not sure about the answer. I know that with w2k pro and XP, sometimes after a repair reinstall, one cannot access existing data files, even as administrator and with the same password for logging on. So the problem may well arise.
The solution for that is for the new administrator to take ownership of the file.
I once read that the reason is, the existing files' permissions are attached to the owner who is identified not only by the name of the user account (such as administrator), but also by the Computer name (which is created when you first installed the OS - one suggested by Microsoft - in w2k, my experience is, it always suggests my own name followed by a set of numbers - and I always delete the set of numbers).
For example, in w2k if you right click a data file, Properties, Security tab, Advanced, then click the Owner tab, the cuurent owner of this item is listed as [Computer name/Administrator].
When you reinstall the OS (even with a repair reinstall), a new Computer name is generated (usually with a different set of numbers). So the administrator account becomes [new computer name/administrator], who according to these OS's, is a different user account and person.
I haven't verified this myself, but it seems to make sense.
When you install xp pro, when prompted for the computer name, try using the same name as before. You can double check the existing computer name by right clicking My Computer, Properties, Network Identification tab, Properties. [You can also change the computer name to a more user-friendly one, which requires a restart.] Actually the name that appears in front of the user account in the Owner tab is the the computer name.
If the problem does still arise, just take ownership of the files or folders in XP (or the entire partition). XP pro may have a slightly different route to get to the Owner tab, but you'll get there.