Perhaps it's showing up in your System Restore points or perhaps an external drive?
Does it tell you which location it is quarantined from?
What operating system and service pack is this?
Try a few tools in the last link in my Signature below. Malwarebytes Free for instance.
The operating system is Windows 7 Home Premium-Svc pk 1. Quarintined from C: my main HD.
Does it say what location exactly?
Try the above suggestions anyway.
The Medfos group of Trojans are well-known and McAfee should be able to remove them. However, you may have other malware which is causing you to become re-infected after the Trojan is removed.
The McAfee advice is to disable System Restore and then to run a full scan (with the latest DAT, if you're not on Security Center v12.1 or later). Remember to re-enable System Restore afterwards.
Microsoft are somewhat more informative about this infection. The two main variants (A and B) make changes to Firefox and Chrome respectively, inserting malicious add-ons to Firefox and an extension to Chrome which have to be removed. See the two Encyclopedia articles below, which show you what to look for.
If McAfee doesn't remove it, try the Microsoft Safety Scanner (link at the bottom of the page on those two articles) which Microsoft say clears it.
If both McAfee and Microsoft fail to clear the infection we're probably looking at a rootkit. There is a detailed article in a malwaretips.com blog which uses about 6 or 8 different programs to get rid of a rootkit associated with medfos. It took one person about 8 hours to work through it all, so I'll leave that one until last.