While one can always easily uninstall Microsoft Security Essentials from the Control Panel, should you find that your are unable to uninstall Microsoft Security Essentials, you may try to uninstall it manually or by using a Microsoft Fix it solution.
To know about the detailed steps required to uninstall MSE manually visit Microsoft KB2435760.
Important: But do check up the version of MSE installed on your computer first and do not use this KB articles for Beta versions or versions that begin with the numeral 2.
Alternatively, if you find this task too daunting, simply download and use Microsoft Fix It 50535 to let it force uninstall Microsoft Security Essentials for you.
This article Microsoft Security Essentials Restore Tool might also interest you!