You can get a virus on Windows RT, but it would have to specifically be made for Windows RT and its much more secure environment. If there was an exploit and a virus maker saw the market... perhaps. To date there is no confirmed reports.
You could also get a file downloaded or transferred from another computer that contains a virus for another OS (Windows, Mac). Defender would want to clean if possible or then delete/quarantine so you didn't spread the virus to someone who could get infected.
But also extremely likely is as described above, a fake alert from a website. A real alert I imagine would throw you back to the desktop (though I have not tested this, I just don't think defender as a Metro counterpart yet). Just keep in mind on any computer you are, virus alerts from Defender or Norton or any AV software comes from that software in a separate desktop window and app as shown in the taskbar. It is never contained in a browser.