This stuff happens all the time. To be technical, it's not a virus, but mal-ware. Your pc is fine, but it tries to create a panic so you'll buy the software and then it *magically* cleans whatever you were supposedly infected with.

I have never liked McAfee, and for years I have run Avast! on mine and all my family's computers. Best part is, it's legally free. Coming from an IT Admin, here's my advice (but you don't have to take it of course).

Uninstall Mcafee first. Running 2 antivirus programs at once is a no-no. It slows your computer to a crawl and they can conflict with each other.
Next, download Avast! Free Home Edition. a quick Google search will get you to the proper site. Install Avast!, and when it is finished installing, it will ask if you want to schedule a boot time scan. Say yes. This will scan the machine before anything loads up into its memory when Windows loads.
Once you finish installing it and choose boot time scan, it will want to restart. Go ahead and it will run the scan. Any files it comes back with saying are infected, it will prompt you to delete them.

You may need to run it twice to get everything Avast! can pick up. After that, let me know if anything else is still iffy. May need to manually delete some registry files if it's one of the nastier ones.

Sorry for the novel, just trying to be detailed, since I don't know your pc knowledge.