Help - can't connect from Vista to XP with admin account

[rowan.bradley]

I have two PCs sitting on my desk, one running Vista Business, the other XP Pro. I have adminsitrator accounts on both of them. They are both connected to the same LAN. I just want to copy some files from one to the other. Currently if I open Windows Explorer on the Vista machine and type \\PC185 in the address bar, it asks me for a logon, and when I type a username (a local user on PC185, with administrator rights, and with no domain account) and password, it tells me:
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Open Folder
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\\PC185 is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions.There are currently no logon servers available to service the logon request.
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Firtly, I am the freaking adminsitrator!
Secondly, what does "no logon servers available" mean? This is a local user, so surely the computer only needs itself - it doesn't need any other server? I can ping the computer fine, so it is connected...
Thirdly, what more permission does it need, other than an administrator account that should have permission to do everything?
Fourthly, "don't have permission" and "no logon server" seem to be conflicting. If there's no logon server, how does it know whether I've got permission or not? Is it complaining that it can't find out who this user is, or that this user doesn't have permission to access this computer?

It would be great to get some suggestions, or explanation, or ideas for more investigations I can do about what's going on here, because it's driving me mad...

Thanks - Rowan