you don't want that. I work for corporations using both Windows "PC's" and Macs and they are intergalactically different in how to approach them from a software issue. There are NO Mac virus in the wild - none - any credible securities lab in the world will attest to that. If you start putting "Windows credible" AV product on your mac you will seriously screw up that Mac. any 3rd party cleanup garbage and again your screwed as well. There are about 2 3rd party packages that are any good and Bob mentioned them both. NEVER use a windows mindset to administrate and protect your Mac. Do not use your mac to detect windows virus (2nd defense - horrifyingly bad idea) and do not use AV on a Mac which is totally slap-dash crap from vendors who would rather make up stats and cause you problems to sell you their useless garbage or add space and give you software that has been documented on these forums ad nauseam to cause problems and fix nothing.
Best software defense: Keep your system up-to-date with patches from Apple.
for now; force quit the browser.
How to force an app to quit - Apple Support
if it's Safari you need to relaunch it holding down the SHIFT key, if it's something else you should check the support page on how to do that mac. once you do that follow bobs advice to the letter and hold of on the PC Wiz kid before you get yourself into a Windows quagmire.
if you cant get the system to respond to anything see these instructions:
Disconnect all external devices (except keyboard, mouse or video if you are not on a a laptop, imac, etc.)
reset the smc
reset the NVRAM
How to reset NVRAM on your Mac - Apple Support
then boot into safe mode
then reboot normally with devices disconnected - then proceed with Bob's instructions if you have not done so already.