Inability to shut down can be a problem with a number of different resolutions depending on the operating system, etc. Whenever posting questions on these forums, please give us as much information as possible about your computer. Depending on your issue, we might need the operating system, processor speed, amount of RAM installed, brand name of the computer, (if there is one), and any EXACT error messages you are receiving, and in your case, what have you done to attempt to fix the problem. The more information you give us, the better informed our answer will be.
In the meantime, start by following ALL of the steps below:
First, scan for spyware by downloading, installing, updating, then running ALL of the free spyware removal tools in the links below: