Missing memory space on my laptop

My laptop is really slow lately. It even came to a point where it’s really annoying. So then I tried to scan it online for viruses but online scan finds nothing except for some Adware. But when I check the available space on my hard drive it’s just showing me 7GB, I check all files stored on my computer even the application and it showing me 8gb. I am clueless on what is happening on my laptop. Does it have something to do with the free space? Please help!
