XP Home/Pro logging in automatically

I have a strange thing happening to my 2 home PC's

One is Home, one is PRO, they are networked, and sharing
a Broadband connection (on the Pro PC)
So, the Pro machine is turned on for the modem connection
to be active, I can then login the other PC (Home edition)

If I leave either unattended, and come back, I have gone
to the login screen, but it can tell me other users are
logged in, and nobody has been near them.

Anyone experienced this?

Both have SP1 loaded, and also running Norton Security.

Thanks

Neil