Permission Problems - Can't open MS Word files shared on Mac Desktop

This is a problem I haven't experinced before.
All of our computers are shared on a local network. For some reason I'm having problems sharing and opening files between my PC and Mac. I can share a file with the PC from the Mac without issues, but can't open files without manually changing the permissions for every single file from the PC to the Mac.

3.2 GHz Quad - 10.8.2 sharing with XP

Thanks