try creating a new non-admin account from manage->users and groups (right click on My Computer-> Manage),
before trying to log into it
- boot your system in safe mode with command prompt
- log in as Administrator,
- in the dos console write "regedit" (without the quotes).
- check your registry keys under "HKEY_LOCAL_MACHINE / Software / Microsoft / Windows / Current_Version / Run"
- delete any suspicious keys
- reboot your computer and voila.
i guess some program that is executed on login time is logging you out. so you should search for such program.
Or your windows could have crashed somehow, maby you deleted some system files...
Good Luck