How do I setup a new machine that came with Windows XP Professional
SP2 installed so that it has access to the existing workgroup?

I made it be in the proper workgroup, made the administrator
password the same as on the Windows XP Professional SP1 systems,
and am using the same username and password on all of the
machines.

I disabled the firewall.

The SP2 machine can't even reach the directories that
it is sharing. For example, D: is shared as DXP1D,
but both \\Dxp1 and \\DXP1 get no access permitted
errors.