If I have the LAN cable disconnected the following strange thing happens:
Windows starts up normal and the logon screen appears. When I then login I immediately get the Logoff screen and windows logs me off. I can then logon again and it logs me off .......
If I then connect the LAN cable and try to logon same as above happens
If I start the computer with LAN cable connected from the beginning all is ok
It sounds like windows is trying to log you onto a domain and not finding it as the LAN cable is disconnected. You need to log on with the cable attached and check your connection settings. You don't say in your original post whether this is a work machine or home. If work then leave it plugged in. If home then do you have admin rights? can you change settings? Is the machine running a logon script such as XPtweakUI autologon?
We need more information about what happens when the machine is connected to go any further.
Alternatively, I've seen this on a hacked XP install that has been WinUpdatePatched to remove the hack and now wants to try and activate/validate outside the normal activation period.