Does anyone know a good antivirus for the computer?

AVG serves me well. And it's free.
https://download.cnet.com/AVG-AntiVirus-Free/3000-2239_4-10320142.html

To me it more likely to be a disk problem then a virus but I would suggest in the future to see about getting the free version of sandboxie to run your browser in to keep any nasty junk on the internet out of your computer in the first place.

Next run the os check disk program found under tools menus for your disk drive.

There is also, a very expense program, that I been running it for years and loss counts of how many times it had fix hard drives problems by the name of spinrite from GRC that you might consider buying.

It may be a virus or it could just be life. All computers run slower, when you first get them they're state of the art, but shortly after that you start downloading more and more upgrades and your computer gets slower and slower.

I use McAfee AntiVirus Plus. Despite the name, it includes a full firewall alongside its antivirus functions.
http://www.pcmag.com/review/327836/mcafee-antivirus-plus

https://www.youtube.com/watch?v=bKgf5PaBzyg

Norton AntiVirus Basic is a top quality malware hunter which can automatically protect your PC all on its own - that's a given. I personally used this antivirus it's running accurately

Yes, but Norton/Symantec is opposed to civil liberties. It's not nice to give financial support to corporations who think that your civil liberties should be violated.