There is a lot of anti-spyware available on the net that are just fraud. They offer free scanning of your computer. But all they do is put spywares on your computer. Then they would offer you to buy their program to just clean the spyware they have just put on your computer.
Trend Micro is a good anti-spyware. I've been using it for almost a year now, and I haven't got any problems so far. I also consumes less system resource from my computer.
They show fake popups that say that my PC is infected with a virus and when you click it, you'll find it an affiliate link. Everyone should ignore such popups and banners.
Yes these spy ware scanning programmes are themselves the host for Spyware / Adware ..... and even technically speaking it not possible to scan for spyware online .... so even if have a professional company claiming to do a online scan ... never believe ... just give it a leave !!!
Fact : Spyware can never be scanned online ... so there is nothing like a online scan for em' .... so if u ever come across such a ad or something just ignore it ...!!!