A program or set of programs which hides itself by subverting or evading the computer's security mechanisms, then allows remote users to secretly control the computer's operating system.
Additional Details
Rootkit:W32/Agent.UI creates a device named "\Device\IWEUFHKSEF" and sets up symbolic link named "\??\IWEUFHKSEF" to accept control codes from usermode programs.
It removes all hooked addresses corresponding to all NTFuntions (which are implemented in Ntoskrnl.exe), then restores them to their original values.