The "All Powerful Bug"

One thing I'll say for this business: It's never boreing!

I am out in one of our field offices, and a user says he is having issues with his PC:

-random web pages come up rather than what he is looking for.
-McAfee seems to be running, but he cannot get it to do an on-demand scan.
-system seems a little slow.

So I come in and:

-Clean out unneeded software.
-look for startup items that look suspicious and disable them
-Install "Malwarebytes" and begin a scan.

As soon as it starts, it gets 2 seconds into the scan, and closes. When I try to start it up again, it says that Windows cannot access the device or file or item and that I may not have permission.

I check the permissions, make sure that I (The administrator) am the owner and have full permission for the file and directory. This is when I notice that the malewarebytes exe file (when I right click and select properties) lists as an MSDOS file. Once I take ownership of the file and verify permissions, it now lists as a regular Windows .exe file.

So, I try running it again, with the same results, and yes, now it is a MSDOS file again -like the file is being hijacked.

I try the same thing in save mode: same results
I go into msconfig and turn off all services except Microsoft: same results.

Then, I try another "Bug Hunt" software program, and it does the same thing again.

Whatever it is, it seems to be selectivly disableing anything that can find and remove it: -I have never run across something like this. My usual fix with a situation like this is to replace the machine, and then wipe the drive on reload on the proble machine. But in this case, I can't.

Please advise!

Thanks In advance.