Is there a way in XP to make a particular domain the default in the logon dialogue box no matter which domain was logged on to last?

From my reading of REGENTRY and: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/security/Security/msginadll_features.asp your answer is no. DefaultDomainName is used when you utilize Auto Admin Logon. The domain name you logged into is written in the registry after every successful logon. There's nothing I could see in gpedit.msc either.

Although I can not vouch for the effect on your system, you could try changing the permission on the following key to Read for the System and remove all other users except one that has Full Control. This would in effect keep the settings the same every time you log in. If you login with the one account that has Full Control, you can of course change the settings back the way they were. Always make a note of the current Permissions before you change them! [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon]