Have you identified the process/es that are running when the xpantimalware thing pops up?
You need to look at the process tree in the Task Manager and find out what it is by ending suspicious looking processes until the pop ups disappear, identify the .exe process and make a note of it, then i'd suggest booting into safe mode and running the malwarebytes software from there.
Once that is done, before you boot back into normal mode, search the computer for any reference to that process, you may find left over bits and bobs in the C/Windows/Prefetch folder, and a search of the registry for the process name may reveal some left over keys that malwarebytes didn't pick up.
You have to clean it out completely and utterly, just deleting things won't help, it won't be in any of the normal places like program files, probably not even in your local internet files, it's far more insidious than that.
You need to be very methodical. In fact, if you just start deleting things you are likely to simply render your PC inoperable before you solve the problem.