I know that this works on a 32 bit install since that's what I am using, but do not know about a 64 bit install.

Windows 7 and Vista use a protocol known as  a "Link-Layer Topology Discovery (LLTD) Responder" component.  The LLTD component is not preinstalled in XP. 

Although XP can be successfully networked with Vista and Windows 7 without it, I found that it has made the experience easier and more reliable to install LLTD to XP. 
What it does is permit the XP computers to be readily discovered on the network and "show up" in the network map (Network Folder) in Vista and Windows 7, provided that you are using the same Workgroup name and have File and Printer sharing enabled.

There are 2 different means to achieve this, depending on what Service Pack your install of XP is using.

If you are running XP with SP2, then you can download and install the LLTD component here:  http://www.microsoft.com/Downloads/details.aspx?familyid=4F01A31D-EE46-481E-BA11-37F485FA34EA&displaylang=en]http://www.microsoft.com/Downloads/details...;displaylang=en

If you are running XP with SP3, then a Hotfix is needed which can be downloaded here:  http://support.microsoft.com/kb/922120]http://support.microsoft.com/kb/922120

To check that each of your computers has the LLTD protocol running, in the Networking Folder on each computer, right click the networking adapter that you use and choose Properties.  Look in the list for "Link-Layer Topology Discovery Responder". 
If it isn't listed, then click Add, then Protocol.  Highlight Microsoft, then click the LLTD entry.  If it's not there, then you are on an XP machine and it needs to be installed.

Reboot after installing the LLTD component.