Viagra offer

By accident yesterday, I clicked into a website where an alert all of a sudden came up and told me that the website contained a ton of viruses. Well the computer was fine but when I turned it back on last night, I couldn’t even go onto any websites because I’m assuming this fake virus protection downloaded itself on my computer. It claims every program is infected like when I click into my AVAST virus protection, it shuts down because this program that downloaded itself onto my computer claims it’s a virus which it obviously isn’t. It also constantly sends me to either a Viagra website or a porn website. It’s so unbelievably irritating. I can’t even download some other free virus protection from a website because when I try to go to a website it automatically goes to a page that says something like AntiVirus Pro. Can anyone help me out here? I’d appreciate it a lot!