Press Control-Alt-Delete twice to get the old login screen and use your account name and password to login.
Such pests can do a lot of damage so I'll just write yes, that can happen.
I am using Windows XP professional SP1. Recently a spyware effected my PC. I used some software to remove it. It was ok for a few days. Later I found that when the windows starts up, it shows the welcome screen but the user accounts have totally disappeared. So there is no way I can login. I used a recovery CD to recover the OS. It was ok for sometime. After a few days the same problem showed up. I had created 2 accounts under administrator group. I do not know if the spyware is causing this. Please advise.