Hi, I suggest you to install BitDefender 60 Virus Scanner, because it is a very quick and light scanner, and it runs in background, so it doesn't slow down the PC.
Then you can install Auslogics Disk Defrag to improve the speed of your PC, so you can install a backup solution, such as Iperius Backup Free.
In addition you can install Collusion extension, which allows you to know which websites are tracking you, and this is very important.
So, you can install MPC-HC, which is a very good and fast media-player, and I suggest you to install a second browser.
Thanks for sharing it!!