HI and Welcome to Apple Discussions...
The pop up you saw just trying to get you to purchase their software. Ignore it. If you are using Safari, go to Safari in the Menu Bar, then click: Block Pop Up Windows.
There are no known Mac viruses. But since you are running Windows, you need anti-virus software.
Go here and read.
http://www.software.com/view/9038/62/norton-antivirus-dual-protection-for-mac/
I would never suggest Norton A/V software for the Mac, however, this software offers protection technology and anti-virus for Mac users running Boot Camp or Windows virtualization software.
ClamXav is strictly anti-virus software for the Mac.
Carolyn 🙂