PLEASE HELP! Windows 7 freezes at logon screen!

Mi computer running Windows 7 64 bit starts up to the logon screen, and then either freezes with the cursor and background, or the background and please wait. Once it got to the logon icons and I got to the user password but then it froze. WHAT DO I DO?

Also sometimes I try to start it and the lights and fan turn on, then it shuts off, then it starts and I am able to try stuff in the F8 menu.