This is definitely not the work of any kind of malware, but something very strange is going on.
It is normal for there to be an invisible file called mach_kernel at the root level of your hard drive, alongside the Applications and System folders. You cannot normally see it, but it should be there. There are a couple questions that spring to mind: do you still have a mach_kernel file there, and how did the item in your Documents folder get there?
To answer the first question, open the Terminal (found in the Utilities folder inside the Applications folder) and enter the following command:
ls -al /m*
The results should look something like this:
Hyperion:~ thomas$ ls -al /m*
-rw-r--r--@ 1 root wheel 8244640 Jul 29 19:34 /mach_kernel
If you see that, you still have that file in its proper place. There's no immediate problem beyond figuring out how the one in your Documents folder got there. If you instead see something like this:
Hyperion:~ thomas$ ls -al /m*
ls: /m*: No such file or directory
...that means that the file has somehow been moved into the Documents folder, which could cause serious problems. If this is the case, you need to run the following command in the Terminal (preferably, copy and paste it):
sudo mv ~/Documents/mach_kernel /mach_kernel
Do not run that command if you already have a mach_kernel file at the root of your hard drive, according to the test above! Also do not be surprised when you are asked for your user password, and when nothing shows up when you type it. That is normal. This also means that you must be logged into an administrator account... if the user account in question is not an admin, which will result in the above command giving an error message, then post back for further instructions.
Once you're sure you've got the proper mach_kernel file where it's supposed to be, we can worry about the question of what happened.
