Why Not Be Able To Access From Another Pc?

You should see in your LAN card properties if 'client for Microsoft networks and file and printer sharing' is enabled or not. I think the file sharing is not enabled that's why the computer is asking user name and password. You can make administrator account on all the computers with the same password. Whenever you will be asked the user name and password, give the administrator password to access the computer. Guest account has a limited functionality. You must make an administrator account from the control panel.

Whenever you are using windows xp in one workgroup or same network, you should have access to all the three PCs. If you are unable to connect/share files/accessing through Remote Desktop there might the following reasons :

1. Network not configured properly.
2. User Rights to access the network.
3. Local Firewall enabled on the PCs.
4. Disabled Remote connectivity property to allow the users.

In the First case, check the IP Addressess of the PCs that all are PCs should be in the same network, check the subnet.. Try to PING the PCs each other.

In the Second case you said, you are using Guest account.  Join the Guest account in Administrators group and try. Otherwise create a new and join the User into Administrators Group.

In the Third case, check whether the Firewall is disabled or not. Please change it to OFF.

In the Fourth case, Check the option "Allow users to connect remotely to this computer". in 'My Computer' properties.