You definitely got scammed. The popup probably wasn't from McAfee as their protection includes a firewall and they wouldn't know if there was an issue with your computer. Not only should you dispute the charges, now you must consider everything on your computer has been compromised. Passwords, PIN numbers, financial account information. A clean install of Windows is the best option and what I would do myself. See the following and decide if you want to tempt fate not knowing what they may have installed on your computer.
If that popup froze your screen, all of the following apply, and very well may even if it didn't.
From Quietman, see: I have been hacked...What should I do?
And this from the FTC:
Tech Support Scams
https://www.consumer.ftc.gov/articles/0346-tech-support-scams
Next, follow the instructions here even if the pop up is gone.
http://malwaretips.com/blogs/remove-tech-support-scam-popups/
Also run this tool.
SuperAntiSpyware (Free version)
http://www.superantispyware.com/
Please read:
PSA: Tech Support Scams Pop-Ups on the Rise
https://blog.malwarebytes.org/fraud-scam/2014/11/psa-tech-support-scams-pop-ups-on-the-rise/
Breaking down a notably sophisticated tech support scam M.O.