virtualbox.org

Is there a way to allow an XP guest to run the IPX protocol in a Win7 Host. Native XP supports IPX, but Win7 does not.

I cannot think of any reason why it would not work. The guest has its own networking software ; it does not use the host's.
What other IPX device are you trying to connect to? Where is it and how is it networked to this machine?

What networking mode do you use? Only bridged would work for sure in this case.

IPX has been discussed several times in the past. Try Googling for "IPX site:forums.virtualbox.org".