See Help and Support and search for "take ownership" and once you have ownership, grant access to the other user. Note, any Administrator can take ownership of files/folders, by default.
HOW TO: Take Ownership of a File or Folder in Windows XP
http://support.microsoft.com/?id=308421
XP PRO: In Windows Explorer, go to Tools, Folder Options, View and uncheck Use Simple File Sharing. Now, when you right click on a drive, folder or file (on an NTFS partition) and select Properties, you'll see a Security tab. Here you can assign or deny permissions based on user name or user group membership.
XP Home: By default, you can only make files and folders under My Documents "private". This is done by right clicking a folder or file and selecting Properties, Sharing. To change the permissions on other folders, you need to boot the computer to Safe Mode and log in on the built in Administrator account. In this mode, you'll see the Security tab in Properties, and you can assign permissions based on user name or group membership.
How Do I Get the Security Tab in Properties - XP Home (makes the Security tab appear outside of Safe Mode)
http://www.dougknox.com, Win XP Tips section
HOW TO: Set, View, Change, or Remove File and Folder Permissions in Windows XP
http://support.microsoft.com/default.aspx?scid=kb;en-us;308418
HOW TO: Set, View, Change, or Remove Special Permissions for Files and Folders
http://support.microsoft.com/default.aspx?scid=kb;EN-US;308419
HOW TO: Disable Simplified Sharing and Password-Protect a Shared Folder in Windows XP
http://support.microsoft.com/default.aspx?scid=kb;EN-US;307874