Spyware probs

Hi, I have a problem which I don't know how to rectify, I switched on my laptop today and I had a message saying that there were virus, trojan etc etc on my system. I had a mad panic and downloaded the software that was suggested in the message to rectify my problem but have now found that I have been duped somewhat to purchasing a security package from company called Security Tool $49.95 minimum pyt!! (I have McAfee already installed! I have Windows XP) I have tried to find program to remove but cannot and I am now getting these blasted Security Tool alerts every few seconds saying that harmful software detected etc etc. Help needed please somebody!! what can I do please