My computer is acting weird.

Oneday i was on my pc doing some work. I turn it off.

Next day turn it on goes past the XP loading bar and shows a classic type logon screen where you enter your username and password manually. I dont know why :S becuase i never selected it. I then try logging on it automatically logs me out. I then restart the pc and it then says cant not find userprofile and a dos box with loginui.exe flashes about :S, I then reinstalled xp two times and the same problem occurs or something simillar as soon as i restart the pc. Please help me!!!