I would guess it's because the Win7 user is obviously different than the XP user. If you had password protected or encrypted the files under XP then it might freak out a little when you try to access them with a new user account from a new OS.
Right click on a folder, click Properties, then go to the Security tab. Make sure that your current account has all read and write privileges turned on.
If that fails, I would back everything up and reformat the drive (if that's an option).