For what it's worth, a year or two ago I had the reverse problem, Win 7 wouldn't shut down. After lots of messing about I did what mixture has suggested and tried booting into safe mode, from which it shut down cleanly.
I updated/re-installed some drivers (one at a time followed by a full Windows start-up and shutdown to check if the problem was still there), and found out a corrupt graphics driver was causing the problem.
Could be anything though! Good luck.