It must be something more than just QuestScan. QuestScan is merely an adware and it wouldn't prevent MBAM from running.
Please download and run the below tool named Rkill (courtesy of BleepingComputer.com)
which may help allow other programs to run.
There are 4 different versions. If one of them won't run then download and try to run the other one.
Vista and Win7 users need to right click Rkill and choose Run as Administrator
You only need to get one of these to run, not all of them. You may get warnings from your antivirus about this tool, ignore them or shutdown your antivirus.
* Double-click on the Rkill desktop icon to run the tool.
* If using Vista or Windows 7 right-click on it and choose Run As Administrator
* A black DOS box
will briefly flash and then disappear. This is normal and indicates the tool ran successfully.
* If not, delete the file, then download and use the one provided in Link 2
* If it does not work, repeat the process and attempt to use one of the remaining links until the tool runs.
* Do not reboot until instructed.
* If the tool does not run from any of the links provided, please let me know.
See, if you can run MBAM now.