My mac has a virus?

I have a 1 year old MacBook. All the people I know who have Mac's were telling me I don't need virus protection. So I was surfing the net using Firefox when a warning went on saying my Firefox has detected a very dangerous virus (spyware) as well as a Worm and a Trojan in my computer. It gave me a link to an anti virus program to download. It said it got rid of the Trojan but the spyware virus could not be removed. After this I went back into my computer and saw that my master lock had been turned off. I put it back on and it encrypted my files once again.

Have I made a mistake encrypting my files now, with this supposed virus in my computer?

What would you do in this situation?

My mac has a virus?virus removal

i will download anti spy to get rid of that spy

My mac has a virus?anti virus protection

First of all, I will download an antivirus which can use in MAC, such as AVAST. Then, I will buy a recovery software.
Try with Spybot it's freeware it removes almost all spyware. you can get it in http://www.download.com/.....