Thanks, but that does not help - I have 'Guest' disabled on "Check System Preferences>Users & Groups>Login Items".
The issue is this dialog box that continuously forces itself into the centre of the screen. It seems to be a problem with just this laptop - "Nikki-PC" and not any other of the devices on the network
At the moment it means that if this laptop - "Nikki-PC" - is beeing used on the network, my MacBook Pro is unusable.
Yes, for some reason your Mac is trying to join/access that PC for some reason, what Sharing is enabled on that PC?
Did you find nothing in Login Items? I feel there is something on your Mac.
To find out if it's system wide or user specific, try this...
Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.
Does it work OK in the new account?