There are no viruses that can infect Mac OS X. If you've suddenly gotten a "report" of viruses from "Mac Defender", "Apple Security Center" or anything of that nature, it's a fake, designed to scare you into purchasing the scammers fake software. DO NOT download anything, or provide any sort of personal data, particularly a credit card.
If you did fall for this trick, Apple has information and instructions for removal:
For additional information, see also:
If you do a forum search on Mac Defender, Mac Protector or Apple Security Center (this fake has a lot of different names) you will also find a lot of threads about this.
I'm pretty sure you're dealing with a group of trojan nuisance malware called MacDefender, MacGuard, MacProtecter, etc. You'll be fine. Here Apple's advice on this garbage:
So glad you're being really helpful here, but you never want to put a live download link here,
even to Apple downloads.
Here's the right link for this.
People can read about it, and download it if they like: