In case of a badly infected system, you may want to consider using an AV scanner totally outside of the operating system.
Note: all of these linked items are free of cost.
Kaspersky Rescue Disk 10
http://support.kaspersky.com/viruses/rescuedisk
Dr. Web LiveCD-LiveUSB 9.0.0 (03/11/2016)
http://www.majorgeeks.com/files/details/dr_web_livecd_liveusb.html
Alternative: create a bootable USB 3.0 thumb drive (32 Gb or larger) as a "Windows to Go" operating system, onto which would also be installed several AV/anti-malware scanners.
Aomei Partition Assistant 6.0
http://www.majorgeeks.com/files/details/aomei_partition_assistant.html
This free software allows creation of the bootable Win-to-Go USB thumb drive.
This anti-spyware scanner is especially good for finding browser cookie spyware:
SUPERAntiSpyware 6.0.1216
http://www.majorgeeks.com/files/details/superantispyware_free.html