Using a Router you can share files from any computer on the network.
You should have no problems sharing fines between Vista, XP and a MAC on the same network.
Ensure the files are in the shared folder, that the computers are switched on and on the same Workgroup/network. The Networking wizard setup in Vista and XP is probably the easiest way to go through the setup.
Alternatively, you can use something like a Network Attached Disk, which means you can store files on a box that sits on the network, meaning you do not need the other PCs on to access the shared files.
I use a DLINK DNS-343 to do this.
Mark