Files/Folders not Accessible

I have a Maxtor portable HD. Use it on my home system (Stand-Alone) WINXP PRO), at work (Networked WINXP PRO), and at school (Either a MAC G-5 or Netorked WINXP PRO). Had to reformat it after a corruption due to improper dismount from a MAC. Did that on my work system (from FAT32 to NTFS). Once I did the reformat and restored my files from backup, many were not accessible anymore at all locations other than my work system... claims I do not have privilages.

Sooo. At work I attempted to gain ownership through security settings.
What do I need to do to make all files and folders completely accessible from all machines?

PLEAE HELP... I'm not and expert, but not a moron either.