combofix usually finished the job, but we didn't use it often because it had a strangely nasty habit of loving with certain PCs...
Just as Trymos said, ComboFix is a bit like the Intercontinental Ballistic Missile of antivirus software. It's designed as a last resort and is generally only used for the most stubborn viruses, which usually are unresearched at the point when you're forced to use CF.
It has very strict definitions on what "safe" files are and will trigger upon anything with even the slightest curiosity about it.
I
never advise Factory Resetting, unless there's proof of something absolutely, catastrophically wrong with the install (such as annihilating your registry, as one idiot did to his poor PC). I actually had one cunt recommend me do a reinstall of Windows because a single GTA V mod wouldn't work. The fact of the matter is that if anything is able to put a permanent dent in a Windows Vista+ installation, it's probably not going to go away with a reinstall (rootkits are absolutely horrific little buggers).
If you have an SSD, do not run a disk defragmentation program, unless you want your SSD to die much quicker. CCleaner is good for cleaning out temp files, but it's usually best NOT to run the Registry Cleaner, as there's little to no performance benefit and may actually cause damage to the system depending on what it picks up.
As bestguy22 said, assuming Windows 10, open up the Task Manager, go to the "Start-up" tab and look very carefully at what applications are starting up on launch and the impact they have on your boot. It's possible that you might have some anti-virus applications set to silently run on start which are bogging the system down. If you do not have Windows 10, you'll need to goto the Run dialog, open up "msconfig" and go to the Startup section. You won't have any data on impact times, so you'll need to basically wing it and start disabling services to test what's bringing the system down.
Ultimately, if you've run 6 different antivirus programs, chances are that you're in a good state. MB, Hitman and ComboFix are extremely solid, not much is getting by them.