Log on screen screen

Hello, i have a unique problem with this dell machine. i reinstall winXP. After doing the updates from dell and Microsoft i notice the windows login screen change from fast log on switching to the slower logon.( where you have to type the username and password). When I try to change the logon, I got an error saying 'a recent application as disable the windows switching. Remove the application' and off course, it doesn't say which application cause that. I've reinstalled window again, only download service pack 3, with some other necessary drivers (video, network controller) from dell. I got the same problem again. I later roll back service pack 3 by doing a system restore. No dice, Same problem. Please help�.