Username option greyed-out

Hi All I've set up a small wireless network with a 3Com router linked to my desktop (Computer Name=Rome) with a wireless connection to my laptop (Computer NAme=Madrid). Both machines are running Win XP Pro SP2 and I'm able to ping computer-to-computer and computer-to-router with no problems. My desktop can 'see' my laptop and laptop can 'see' my desktop when I select 'view workgroup computers'in My Network Places. Problems become apparent when I try and open/explore my desktop (Madrid) from my laptop (Rome) - I'm asked for a password for the following username: madrid/guest The problem I have is that this username is greyed-out so I'm unable to change it to MY username. I don't have this problems when I try and access my laptop from my desktop. So, to clarify - when I try and connect to my desktop from my laptop I'm asked for a password for a username (madrid/guest) that is greyed out. How do I change this to my primary user account? Somebody please help. Thanks in advance Clueless