Can not visit some sites because of too many ads! Is there any virus in my Mac?
Try BitDefender's freeware tools -  

Adware Removal Tool for Mac
Virus scanner for Mac
Mechanic

The Adware remover you get directly from the link, the others you can get through Apple's App Store.

I would highly recommend keeping the Virus scanner on your machine afterwards &amp; running it periodically.
Macs are not immune to viruses, they're just rare.
The freeware version does not actively scan in the background, you need the paid version for that.

BitDefender is highly rated on official antivirus test sites such as http://www.av-test.org/en/