How to set default login used from one PC to another in a workgrou

I have a network with 4 workstations using a workgroup (no server). One of
the workstation's machine name was changed and now one of the other
workstations cannot connect to it in the My Network Places - Entire Network -
Workgroup interface. I can map a drive letter to the machine using the "Logon
as a different user option" and then it will connect correctly even in the My
Network Places interface, but if I disconnect the mapped drive it goes back
to spitting out a invalid logon message. When I map a drive letter that way,
the drop-down list shows the old machine name, which I can change to the new
name to make the drive mapping work. It appears to want to use the old
original machine name by default whenever there is no active connection to
that PC. None of the other workstations in the workgroup are having this
issue -- its only on this one machine. And the PC I'm trying to connect to
can connect in the other direction just fine.

My question is, where can I change the logon name/pswd a PC will use by
default when you click on one of the computers displayed in the workgroup
folder in My Network Places - Entire Network? The first time you do it a
login window pops up so you can set it, but from that point on it does not
appear.

All PCs in the workgroup are running Windows XP Pro 32-bit + SP1 + SP2 +
SP3. They all have File And Printer Sharing enabled and NetBIOS enabled over
TCP/IP. Firewalls are configured to trust all PCs whose IP address is in the
range our router uses.

Thankyou in advance for any insight you can provide...