How to I allow sharing on XP/vista netoworks without needing to login to "connect"?

I have a home network with all computers connected to a single router. Three of these computers use Windows Vista and three of them use Windows XP. I have managed to get all computers into one workgroup, titled WORKGROUP. The problem is that when I try and access the shared space of another computer from the computer I am on the computer requires me to enter a login and pword to "Connect". This problem arises in only a few combinations; some computers will just allow me to view the shared files and I can't see any differences between the permissions of these two computers. The major problem that arises from this is that one computer does not require a pword for its logon and consequently I should not need to enter a pword to "connect." However, the connection requires a pword. How do I make the computer allow others to connect to it without needing to give a pword?