First of all, try to boot into safe mood without having any problem
because windows may fix an error and then restarts normally. I would
guess you have windows 7. Like other windows software when you boot
your machine keep pressing F8 so you boot into Windows startup menu to
select Safe mode.
If it boots normally. Then you. Have no problem.
Other than that you may want to do a fix or repair installation from
the windows DVD if you have it. Another aprouch I'd to make a backup
right now on a DVD so you won't be lost ever.