when I logon to xp Professional it logs me out

it start to log in then is switches to logging off. I
have tried different users, and all users do the same.
This workstation is part of the domain, I have tried
logging on locally as well as the domain and it does the
same thing every time. Have been able to connect to the
computer over the network and even create a new local
user and it does the same thing. I have checked to see
if there are any weird startup programs in the registery
and or the all users startup. I have scanned the hard
drive for viruses, also looked in the registry for
strange entries.

I don't want to have to reinstall the operating system if
I can help it, but I am at a loss....

Any suggestions

Thanks
Dennis