If you wish to do it right you leave any sorts of security enabled until you have done the usual fixes. If a "common human" comes with trouble like that they usually just never cleaned their Windows so that is what you do first (temp file, registry, disk defrag (the latter only if it's a traditional HDD like in this case, don't defrag an SSD since it leads to unnecesary writes to the flash memory and won't make a difference as an SSD has a built-in optimisation). If it is still problematic, you could take a look at what applications are used, and such like. First things first.
Nobody checked any sources about McAfee and you just believed them instantly. Don't forget she paid for it so at least be honest about that (and offer reinstall or a refund, because it's not your decision what firewall, antivirus and security related browser plugins the client runs. You should do that in counsel with your client if you want to do business in a decent way).
Your client hasn't seemed to care about speed (or any possible lack of it) until her pc started freezing completely, so replacing a full pack containing an antivirus, decent firewall and some browser plugins (safe searching, site advisor) with only a free antivirus seems a bit a bad idea to me, especially when you keep in mind that she is doing finance related work on it. Speed is nice, though not holy.
I am not sure about what kind of price for things like this are usual in the US or where-ever you live, but 75 dollars sounds a bit much to me (IMHO). You just abuse her lack of knowledge about how to maintain a Windows install (i.e. to keep it clean and running smoothly) and since she doesn't have a clue on how relatively easy the problem could have been avoided, she would probably gladly pay whatever you ask. I hope you are not "like that" and don't ask more than 65 dollars or so (aim at 50 bucks IMHO). Else I cannot be blamed of any possible decreases of your karma.
