Windows 7 Sharing Insists On Password

I have two Windows 7 Home Premium machines on the same network, and in the same workgroup for the benefit of an XP machine. For the moment, though, only the Win 7 machines are important. I do not have a homegroup set up at the moment (doing so does not change anything.)

For the purpose of this exercise, I will try to access Machine B from Machine A. (The same thing happens if I try this on Machine A from Machine B.) On Machine A, I click Network, then click on Machine B. It pops up a Windows Security box that insists that I enter a network username and password to log into Machine B, saying "Logon failure: unknown user name or bad password".

I want it to freely share with everyone on the network, just like XP used to do. Before you say it, by the way, password protected sharing is already off on both machines.