I once fixed a PC that wouldn't shutdown due to junk installs and an unhealthy system. Assuming you don't know enough about computers to see this isn't a CPU problem, OR, you don't read enough to see this is a CPU post, it might be too much for you do this unless you drink an Amp energy drink before hand:
Download CCleaner here, on the top right:http://www.filehippo.com/download_ccleaner
Install, then under Cleaner run "analyze" and "run cleaner".
Under Registry, run "Scan for issues" and "Fix selected issues"
Under Tools, "Uninstall", start uninstalling crap you don't want on there.
Do not uninstall programs that have very recognizable names in them.. like microsoft.. or creative.. etc. If you're unsure about it, then google it. Most likely there is some junk program on there that is preventing normal windows usage such as shutting down.
After you've spent a good amount of time on this, go to the Startup section of CCleaner, and turn everything off that you don't want starting up with windows.
After ALL of that, close CCleaner. Try to reboot. If it reboots, then things should start getting better... if it doesnt reboot, then turn the PC off and let it start up, and THEN try to reboot from windows. If it STILL doesn't reboot, then go back into CCleaner, uninstall programs and startup programs and look some more. It's impossible for anyone to tell you exactly what is stopping windows from doing what you tell it to do.
Once you have a rebooting computer, go download Mydefrag here:http://www.mydefrag.com/Manual-DownloadAndInstall.html
Install, run, and do the monthly defrag. It could take 12 hours, or 3 hours.
After all of this, and still no luck, then you might want to consider saving files you don't want to lose, and then reformat.