Attributes change to read-only

I have a fully updated XP Pro system and am running Norton AV. I recently encountered a problem with my directory attributes. They change themselves back to read-only no matter what I do. Any scans I’ve runn come up negative for viri and trojans,and I can’t find anyhting in the registry. If you know what I can change to correct this or what trojan does this please let me know. I am the administrator so should have no rights issues.