Win7 to Win2000 on LAN

I have a Windows 7 professional machine, and I am attempting to connect to 5 different in house computers. All 6 have are connected on a local network via an 8 port 1 GB Netgear switch, and have no Internet access. They all exist in WORKGROUP. Two of the machines are XP, and three are Win 2000 pro.
I am logged on as administrator on all 6 machines, and none have a password.
I can connect to the XP machines with no problem, straight from a WIN7 boot.
To connect to the WIN 2000 machines, following a WIN7 reboot, I am asked to enter a USER name and a PASSWORD. I type in the Username "administrator" with no password, and I am in. Any time after this reboot, no prompt is presented to access the the WIN2000 machines.
My problem is that this only functions with 2 of the 3 WIN2000 machines. The WIN7 machine sees the third WIN2000 machine, however when I attenmpt to access it (it has about 12 shareable folders) I am again asked to enter a user name and password. When I do as I do on the other 2 WIN2000 machines, I get an "Invalid User Name or password" message, and I am done. I could keep trying with the identical results, probably forever.

Every machine, the 2 XP, and the 3 WIN2000 machines permit me to see and access all the other 5 machines (including thew WIN7 machine). I cannot in any way make that WIN7 machine allow me to access the one WIN2000 machine. I know it is not the adaptor card, as I have swapped cards between WIN2000 machines, and the result never changes.
Two of the three WIN2000 machines have SP4, and one indicates that it has no Service Pak whatsoever. However it is one of the machines with SP4 that has the problem.

Any help would be really appriciated.