There are a load of viruses that can easily sneak in beneath a firewall
Spyfalcon is one, I had that 5 times! the only true way of doing it is going to the Sys32 folder and viewing the icons that were least modified and going through each file one by one...Copy the file name and simply paste iot into google and one of the many sites will come back telling you what it is, if it's dangerous etc etc
Then you may have to go through the registry...You may also need a load of tools you can download which will tell you how to do it by booting in SAFE mode and running various programs.
I myself and now happily on a Mac too!!