Vista - Administration

Upon booting up Vista (HP) insted of going to opening Users page my computer
now takes me to a page that asks me to press the Ctrl, Alt and Delete
buttons. After this, it then asks me to enter my administrators name. But
worst of all, from here it will not allow me onto the opening desktop page.
It keeps telling me that my administrators name(s) and passwords are
incorrect - which they are 100% not - before asking me again for the correct
name. Sometimes it trys to open up and says 'welcome' as if it is normally
opening up but then it returns to the Ctrl, Alt and Delete page. I've tried
F8 memory reinstalation - nothing. Anybody any ideas... please.