|Were you using a password for your user in Windows 7? |
If so, you may not be able to do anything with the Windows 7 files, unless you "Take Ownership " of them.
However, I don't know if you can do that with what's built into XP, regarding files made with Windows 7 - you will need an XP CD.
How to take ownership of a file or folder in Windows XP
NOTE: you probably have to reboot the computer before the settings you change actually take effect!