I think my computer has a virus...trojan...something.

I keep getting this pop-up banner saying that my computer is being attacked from the Internet and how should this program (that I downloaded and installed...my bad...but uninstalled) handle it. NOOOOO!! I ran my McAfee virus software that Larry, my sister's boyfriend, put on my computer when he built it, but it never finds anything. Larry said that I should uninstall that and reinstall it from Comcast. Comcast customers get McAfee for free. Since I was only sort of shown what to do with it when he built my computer, I don't really know
how to use it. I hope this works. So far, nothing bad seems to be happening on my computer, except for this pop-up banner every 10 minutes or so.
