I agree with the comment about getting the AV program in advance, and just slapping it onto a USB. That said, just getting onto the net doesn't expose you to viruses. Plus: on the new install, Windows Defender should be running by default, so you will have some protection. The first thing you'll be doing, is installing updates...a rather tedious process that takes HOURS. After that, tho, if you don't have your AV of choice on a stick, then just go to whichever one you've chosen, download it, and finally, let it run a full system scan. It may actually not take all that long, as you don't have that much stuff on there at that point.
Avast has been getting poorer reviews; Avira has been getting better ones recently. I've been...