@earnshaw: By default XP mode is configured for NAT, if your require incoming access to the VM then you need to do as you say.   XP mode is primarily for software compat, and most software only needs outgoing access, so the default NAT'ed configuration is well chosen in my opinion, it is the most likely to work for outgoing access in all situations.

More details here:

http://blogs.technet.com/b/windows_vpc/archive/2009/09/26/networking-and-using-windows-xp-mode.aspx

This is the first search hit for 'xp mode networking' btw.