Do you have a cd or flash drive that you can put the MBAM and SAS on so you can try to install from there instead of downloading it on the suspected infected computer? I am assuming you already tried renaming all the MBAM or SAS setup files to something like explorer.exe or winlogon.exe and then trying the setup? Another excellent scanning program is Dr.Web CureIt which can be downloaded for free from http://www.freedrweb.com/cureit/
One option would be to try to start in safe mode and then try and run SAS or MBAM. Have you tried that yet? You can get into safe mode by restarting your computer and tapping F8 when the computer is starting. If you time it right, you will then see a screen with white text that will allow you to use the arrow keys to select safe mode or safe mode with networking among other options. I would try safe mode with networking first, and if that doesn't work, you should try basic safe mode. Then try to install and run both SAS and MBAM.
Let me know if that works or helps.