Windows Security Essentials re-downloads itself after deletion

Windows Security Essentials re-downloads itself after deletion and I can't stop it!  What is happening?  I am running Windows 7 on a client computer that is connected through a server.  No one else here is running Windows Essentials so I can't see how the program acts on their computers.  I have followed the deletion process Microsoft suggests and the program keeps reinstalling itself.  I want to use another virus protection program but when both are on the machine they of course conflict.