Cannot access remote folders or get past "username/login" prompt

I have two Win 7 x64 machines. They are on the same homegroup and are physically plugged into the same switch.

Each machine can see the other on the network, no problem.

Machine 2 can See and access shared folders on Machine 1, no problem'

Machine 1 can see the Machine 2, but whenever it tries to browse or map a drive on Machine 2, it comes up with a dialog box for Username & Password. Even knowing the exact user name and passwords for Machine 2, cannot get past this dialogue box.

I can Ping both directions using 192.168 addresses. I can ping from Machine 2 to Machine 1 using name. I CANNOT ping or get a response the other way using the Machine 2 name from Machine 1.

Something is up.

No Virus protection or firewall on either machine (disabled).

No known corrections/adjustments to either machine--both relatively new.

Virus checks have been done on Machine 2, but there are some other symptoms that would suggest the virus checker may have missed something at the boot sector level, but no proof for sure. Have run checks with 5 different checkers, all come back negative.

I need Machine 1 to have read/write access to Machine 2. I can do it the other way around, so therefore it should work!

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