Windows Shield Center is a fake tool that infects your computer and pretends to be a legitimate Microsoft product.
It generates fake Microsoft Security Essentials Alerts in order to scare you into thinking your computer is infected, then directs you to buy the full version of this fake tool.
Below are easy to follow Windows Shield Center removal instructions.
Since Windows Shield Center is also a trojan, it does not come with an un-installer.
To make its removal hard on you, it blocks you from running your programs and can also block Internet access.
Therefore,removal of this trojan requires you to work around this blocking ability as follows:

1. In Windows, click on Start and select Run.
   
2. In the field that opens, type 'shell:appdata' without the quotes.
   This will open up your Application Data folder.
   
3. In the folder that opens, locate any executable file with a name made up of random characters, example pdtudw.
   exe.
   Right-click this file and choose 'delete.
   Make sure to not open any sub-folders of Application Data, stay in the main folder.
   The file will be located in the main folder only.
   Don't worry if you do not see the '.
   exe' extension, just make sure it is a file you are selecting, not a folder.
   
4. Once you have successfully deleted the above file, restart your computer.
   Make sure to bookmark this article before you do that so that you can refer back to it.
   Deleting this file will allow you to once again run your programs.
   
5. Start up your anti-virus program and update it.
   Run a full system scan to remove the infection.
   

By following the above steps you should be able to successfully remove Windows Shield Center.
If in case you have any trouble with the above steps, or if your anti-virus is unable to pick up this infection, you can download this tool which can work around Windows Shield Center blocking abilities and remove it.