How to improve computer speed?

Uninstall unnecessary software.
Limit the programs at startup.
Add more RAM to your PC.
Check for spyware and viruses.
Use Disk Cleanup and defragmentation.
Consider a startup SSD.
Take a look at your web browser.

I have anti virus software, would it consume my computer’s space?

It won’t as long as it helps your computer to remove virus.

How is start up SSD?

A boot drive contains the Operating System and OS relative files that help to load the system. It also may contain some user’s data, such as the user profile, installed programs, application files, etc. Therefore, an SSD boot drive is a disk that contains your OS, OS data, and some personal data.

Thank you and also you have posted more about SSD.