Win 7 asking for non-existant password

I have 3 computers on my home network. All are wired via gigabit connections. I have Windows 7 Professional installed on computers 1 & 2 and Windows 7 Home Premium on computer #3. All 3 computers see each other in the network display. All are set up for sharing and all can access the drives on the other computers except computer 2 & 3 cannot access computer #1. All 3 were set up without a password.
When access to computer #1 is attempted from either computer 2 or 3, a popup occurs stating that an invalid user ID or password was entered. In actuality no password was requested nor entered. Accessing the computers #2 & #3 never requires a password. I have tried adding a password to computer #1 but the error persists. I have re-installed Windows on computer #1 making sure no passwords were entered during the install. After installation it still requests a password.
There is one difference between the installs that may be significant. Computers #2 & #3 have OEM versions of Windows 7 while computer #1 has a retail version of Windows 7.
It should also be noted that computer #1 previously had an OEM vrsion of Windows Vista which did not have this issue. It has only begun with the install of Windows 7. Also note that the Windows 7 install was clean not an upgrade.
I am at a loss as to how to resolve this.

Thanks for the suggestion. The problem is that I tried that and it still does not work. This also does not allow me to run without a password.

Yes sharing is turned ON and each computer has a unique name. Furthermore I have verified that the Advanced Settings are the same for all 3 computers.

Yes, Password Protected Sharing is turned off, on all 3 computers.

My husband and I are having the exact issue. When ever I try to look into my laptop (pc running windows xp) this machine running Windows 7 Prof. asks for a password. Same thing when trying to get into my Mac.

I've done everything you've suggested to the previous person. Nothing helps.

We just installed Windows 7 Professional and he said he never entered a password for "administrator"