Try entering in safe mode.
Start the computer and keep tapping the F8 key till a menu appears, use the arrow keys to select 'start in safe mode'
If you can get into windows, go to Start>Run and type msconfig, in the window that opens click on the Startup tab and untick all the startup options apart from Systray and Explorer. If you re boot the computer and see if Windows starts. Start ticking the options one-by-one and re booting and see if you can find which program is stopping normal operation.