Hit F8 continuously while it's starting, and you should get into what's called Safe Mode, where only the basic requirements for Windows to run are loaded. It won't be a very good picture and your desktop will be a mess, but don't worry about that for now.
What I think Sandra and myself are thinking is that maybe something which loads at start up is 'hanging', and stopping Windows from starting. If it starts in Safe Mode, this almost proves that theory, and then we have to establish what's causing the trouble, which is why Sandra asked if you have recently installed any new software or hardware.
Sandra may well, of course, be thinking something entirely different!