Hi NanoG,
I've been wresteling with the same problem, except that in my case the XP box received the "logon failure" message, while the Win2K box could acces the XP-shares without any problem.
I came across some rather unrelated message stating something about security settings. I had never heard of this, so out of curiousity, I opened "Settings -> Control Panel -> Administrative Tools -> Local Security Policy" to view and/or modify the security settings.
I found that "Access this computer from the network" under "Local policies -> User rights assignment" was set to "<nothing>". After changing this to "Everyone", and restarting Win2K, my problem was solved. Now my XP box can access the shares of my Win2K box just as good as vice versa.
I also verified this setting on my XP box, and there, a list of user groups was already set up, including the "Everyone" group.
My guess is that chances are big that this will solve your problem to.
And if it doesn't, then maybe this will help some other solution seekers...
Regards,
Karl.