Password protected file sharing in windows XP

Hi

I have 5 pc in my office but I want only two PC to share files with each other. Currently I have a shared folder but everyone in my office have access to those files.
I have enabled simple file sharing in windows xp and I don't know what is the way to protect them. We don't have budget for extra servers or hardware. Also buying a new windows server is also not possible.

Please tell how can we do setup that only required user can access the files.

Thanks in advance!