What do the heuristics options in my Advanced Settings actually affect? And Webroot Infrared?

I want to know what the Heuristics settings on my Webroot Security Plus actually do, and what they affect. I want to understand what my Webroot security system actually does and hpow it works so that I can make the best decisions and keep my computer safe.
 
I have heard that Webroot has a specialty of trying to narrow down what is actually searched so that "idle," or "inactive," or "dormant" files are not scanned - since they aren't actually "doing anything" at the moment.
 
This causes me to be concerened. I see why this would be invented - to speed up efficient scanning - but I want to be absolutely sure.
 
My computer is sluggish, buggy, and acting up recently. I am running the most recent Windows update as of 4/18/2016, I think it is Windows Ten..jpgs and.pngs are disappearing apparently, saying they have zero bytes. I have sentimental photos and don't want them randomly assasinated one by one by some bug or virus.......but Webroot says there is no infection. Sometimes my Internet is stuck in the airplane mode position, and needs a restart to get it unstuck. Programs are "not responding" and freezing up. I run a lot of internet tabs at once, could be the problem, and I have a huge amount of.jpgs, could be the problem. Lots of other little running errors like that, makes me concerned, but nothing overt to say I have a security breach.
 
My University is a tech school and some people hack for fun, Just to mess with people. If I went away to the restroom and they plugged in a thumb drive quickly, how would I know? I am just concerned from a past of using norton and symantec and similar programs, that I can just repetively and futilely "scan" my computer again and again until the cows come home, and each and every time it will report: "12 threats detected, 12 threats removed..." and they will be there again next time too. This indicates to me that at that point the malware/spyware has got my number and has overcome the security program, so that the security program is simply running in circles like a security gaurd locked inside his own office and disconnected from calling for help, while he watches the bank he works at get robbed on the video feeds on his desk.
 
See what I mean?
 
At that point, the computer would slow down and not run pleasantly and smoothly, and it seems this is what is happening now too. My computer is also about three years old, an HP Sleekbook ENVY.
 
Also, Windows defender is "turned off" and I cannot turn it back on. In fact, Windows keeps giving me a popup message every day about how BOTH Webroot and Windows Defender are turned off. Webroot, when opened, says it is on and running fine......Hmmm. Concerning. I go into the Window control panels and hit the button to turn on Windows Defender, and it says this is not possible and that I don't have the authority to do this anyway. So I am concerned, but also chugging on because I have ti keep working on my laptop to finish my schoolwork.
 
I would definitely set Heuristics to maximum if it meant it would be tougher on worms and viruses and be more suspicious of everything, but if it actually increases its "intelligence" in and propensity for "overlooking" files which are "dormant," "inactive," and "harmless at the moment," then I would want to turn heuristics off.
 
See what I mean?
 
I would like an overview and an understanding of how the heuristics of Webroot work.
 
I would also like to know the same information about Webroot Infrared, what it does, how it works, why it is good, and if it would increase the likelihood of some files being "overlooked."
 
Webroot infrared can be turned off in the options panel, and heuristics can be adjusted, increased, decreased, and turned off as well. Clearly the designers concieved of reasons why doing this would be good or necessary, or else they would not have included these on/off features. So what reasons are those?
 
Thansk for your answers,
TylerFinn8