If you have a 32 bit and 64 bit machine, they can't be sharing via the OneCare Printer Sharing feature, though the XP and 32 bit Vista machine may.
Basically, you can manually share the printer using built in Windows functionality for a network.
http://www.microsoft.com/windowsxp/using/networking/expert/honeycutt_july2.mspxhttp://windowshelp.microsoft.com/Windows/en-US/Help/e3faee7d-a7cd-40aa-b240-006dca11dc451033.mspx-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator