What is the best FREE software i can download, to remove viruses, adware, and spy's, from my computer.?

Those damn s are at it again, every time i go on my computer, the damn message saying i have viruses keeps popping up. And its starting to piss me off, Can somone please tell me what kinda of software is downloadable for FREE, and that i can SCAN my computer FREE, and REMOVE them for FREE, because i dont wanna buy one of those programs now. GOD-DAMNIT_STOP POPPING UP. xO