This is the boilerplate diagnosis of your problem Windows 7 or Vista will not shutdown
, it is written for Vista but it applies to Windows 7 too.
You need to track down what component, process, or program is causing the issue. Since there is nothing in the box I would focus on processes first. There are a lot of them so you can STOP them 5 at a time, then do one at a time once (if) you find the offending "group".
For components you can go into device manager and Disable them (not Remove) one at a time. But certainly start with anything connected via USB just by unplugging them.
Do you have any issue with Startup?
Have you made any changes to your system that coincide with the start of the behavior?