Whats the best freeware for Antivirus, Antispy, Antitrojan, and Registry Cleaner for my slow CPU.?

Have used: AVG, AVIRA, ADWARE, CCLEANER, CYBERDEFENDER, EUSING and others. But my comp is still slow. I think I might have downloaded something infected or downloaded some malware and I don't know how to get rid of it.