Is my Webroot being fooled by malware?

Hi, I need some help from you tech-savy saviors. My computer has been randomly moving slow for the last 2-3 months. It's not seriously slow, just annoying. I'm working with a Windows 7 os (and I refuse to upgrade to their crappy new os that has an interface full of apps). Often when I try to open a document or my pictures folder the computer starts to run really slow (the cursor still moves at regular speed). I initially thought it was because I had too many media websites open but that's not the case.
 
I've been scanning my computer constantly but Webroot only found one threat and that was just last week. Usually my scans only last for max. 10 minutes but recently they've gone up to 2 hours. That's only when I scan while my computer is acting funky. I often have to restart the computer and scan again to get it to scan in a resonable amount of time and for my computer to run at its normal processing speed. Yet Webroot still detects nothing. I've used all the troubleshooting scans on my computer but they've found nothing. I get this "powershell stopped working" message often so i'm starting to believe there's malware on my computer that Webroot isn't noticing. What do you guys think? Is that possible or is there some simple solution?