This is about my mothers computer:
Two years ago (or maybe a little longer)I placed a (formatted) hardisk from my computer in my mothers new computer. I didn't know there was a virus still on the harddisk. My mothers computer got extreme slow in just a couple of ours. I removed the placed harddisk but the computer stayed slow. I formatted the main harddisk (many times) used a lot of virusscans. But they end op finding nothing. They just crash or take too long to scan.
I tried everything like fixmbr and formatting over and over.
When i scanned it with MGER It found this file: Library C:\Program (*** hidden *** ) @ C:\WINDOWS\Explorer.EXE 
Also should I tell that I didn't install anything other than the drivers for the computer to function (from the official cd's, MCaffee, but the program slowed everything down to an extreme. I couldn't use any other program so I removed the virusscan again.
Also: when I fixed the MBR a warning said that the MBR was not the standard MBR.
CPU get's high a few times of the day. And keeps that way for ours, while no program is running and no windows update.
I don't know what other detailes to give you. I hope you can help me because I'm kinda desperade.