XP sharing, user name problem

I have a peer-to-peer network of winXP computers.
When I try to access a particular computer I get the "Connect to [computername]" dialog where I need to enter a username and password.
The problem is that the User name part is greyed and the option is "[computername]\Guest"

I want to enable access to only those users who have an account on that computer. When the above dialog appears I want the user to input his/her user name and password to gain access to the share.

Many Thanks,
Rob