Get off the 'net, never allow people (including yourself!) to cucuk their flash drives/mem cards into your system, only install original games bought from trusted shops, use original windows OS and software...
There are plenty reasons for systems becoming sluggish. Post this in the technical problems forum. Provide your system specs too
Anti viruses do use a lot of system resources, especially during full system scans, it doesn't matter whether they're free or paid.
Try these:
- find out more about the slowdown. does it happen all the time? only during specific times e.g boot/shutdown/software updates etc.?
- know your av timetable -when does it updates, runs scans, how it handles threats etc.
Seriously man, you're paying (in this instance) for the av, take your time to set it up to your liking, contact the company to report your problems, get your money's worth!