How can i get rid of this particular spyware, NSIS?

I have spyware on my computer, and its called NSIS media advertisement. it wont go away. i use web root spy sweeper, and it's always updated, and micro tend anti-virus. the spy root sweeper always catches it, quarantines it, and deletes it, but it comes right back. ive searched for it in safe mode and tried to delete it with command prompt, but it just wont work. anyone got info on a simple solution as to how i could possibly destroy this pesky spyware?