Another culprit is actually hardware overheating which can cause the computer to slow down to a crawl as well. My parent's HP computer had all of it's air holes coated in dust.
While I was visiting one day they mentioned they needed a new computer and wanted recommendations. My parents aren't power users so I had to wonder what application they were suddenly running that caused the computer to seem slow?
My first thought was my nieces probably loaded it up with malware from browsing the internet with no concern for where they went.
I had my parents clean the air holes of the computer. Before I left that day I kicked off the defrag program provided with Windows thinking if this doesn't fix it should be a good start.
I finally after ruling out everything else had an a-ha moment and thought to check their CPU and the heatsink was so caked with dust that the heatsink was doing the reverse of it's job and retaining rather than dispersing heat.
Told my parents that it was going to be touch and go. Kind of like a person who gets a severe fever as a child. Might come through with no problems. Might still be broken. Might not be quite right the rest of it's life.
I had them leave it powered off to cool down for at least half an hour. A couple hours later my mom turned it on and it was back to screaming like it was new".
