Windows XP doesn't know who owns the files and folders you're trying to access. When copied them over they had ownerships, and permissions attached to them via NTFS resources. When you reinstalled XP the corresponding entries in the registry were lost, so Windows is clueless.
Even the Administrator cannot access resources he doesn't have permission to. However, he can change the ownership, and thereby gain access. This is what you want to do. Now... Here's the hiccup: Windows XP installs with Simple File Sharing (SFS) as the default permissioning system, and not the detailed Access Control List (ACL) method you want. Disabling SFS will ENABLE ACL, and that will provide the security tab you need. For some reason the ability to change ownerships on sub-folders doesn't seem to work all the time, or on everything. You just have to plod through the dialogues each time you encounter a folder you can't get into. If you still have a problem at this point, BOOT in SAFE MODE
and then you should be in business