WinXP Home and WinXP Pro networked with Windows 7 Pro
How to make this work?

2 WinXP home Pc's
1 WinXP Pro PC
1 Windows 7 Pro (acting as server for files and backups)

I need them all available on the network which is the easy part.
How do I allow only 2 of the PC's (1 XP home & 1 XP Pro) have access to a
certain folder?

This folder is available on the network, but mapped to the two PC's as the
other one shouldn't have access to it.

Currently nobody can access this folder only see that it exists on the
Network.

Any help would be appreciated!