Do I have a virus or not?

After running iAntivirus because I was suddenly told that I had been affected by viruses, the scan tells me that i have been unaffected. Furthermore, a second scan on a different application tells me that I have been infected by FOUR viruses, backdoor, spyware and adware (i think) I don't know what to do, whether I do have a virus or not, I need to get rid of it. They say that Mac rarely gets a virus and so I'm very wary about this. I have not been on any bad sites as such, this was so unexpected and has had me staring at the computer screen for three hours straight. Please help me figure what to do.