i suppose that while booting your welcome screen comes and the computer hangs and You can log in by using Ctr/alt/Del keys.
did you install any new hardware /softwares recently?
if so then some of the settings of the hardware might have changed which prevents normal logon,try reinstalling or uninstalling the new software/drivers.
Try booting from safe mode.
While computer is started,press F8 continuously and try the last best configuration.
If problem is not solved try logging into safe mode in the same menu and use windows restore functionality for restoring the computer into a previously working state.
3.If the problem still continues try to see what are the startup programs that begin with windows startup.You will have to temporarily backup the registry entries for the startup items and try disabling one by one until the list is empty or the problem is solved.If you can get the normal login after disabling any startup program try to change that startup program from the computer or rollback any changes made by that software.