XP will not “run”

My XP machine boots up. Asks me to click on the user I want to log in as. I only have one so I click on it. It starts to load the desktop then just logs off again…within seconds. Brings me back to the screen where I choose the user. When I CTRL-ALT-DEL on that screen, and try to log in as Administrator it tells me somthing about a security violation or something. I hae no clue here. Any help would be appreciated. Thanks.