Help! My computer is crashing! I think it's sick...?

I think I know why my computer is crashing. I was trying to download a video but I didn't realize it was on an unauthorized site. I think I contracted a virus, but before my virus scans can complete their searches, my computer freezes or/and crashes and it's really stressing me out, since I know it's my fault. I have McAfee and Norton, but they aren't killing the virus{es}. It's a new computer, barely a year old. Is there anyone who can help me?