Step 1:
Download the installation file for the free anti-virus solution of your choice but DON'T (initially) double-click to run it. My own preference is to use Avast, which has served me well and generally gets good reviews in the technical press:
https://www.avast.com/en-gb/index
Step 2: Download the Norton Removal Tool:
https://support.norton.com/sp/en/uk/home/current/solutions/kb20080710133834EN_EndUserProfile_en_us
Step 3:
Run the Norton Removal Tool
Step 4:
RESTART YOUR COMPUTER. (IMPORTANT!!!!)
Step 5:
Now double-click on the installation file for your chosen anti-virus program, to run it.
That's basically all that you have to do but I'll add the following:
If you've chosen Avast you'll soon get very, very annoyed with the American woman's voice that periodically announces that Avast has been successfully updated. So let's get rid of her!
Click on the Avast icon at the bottom right of your screen. (You might need to go via 'Show Hidden Icons').
Click on Settings
Click on Sounds
Click to remove the tick alongside Enable Avast Sounds
Click OK
You also need to remember that Norton is an entire
suite of security programs and you've only replaced just one part of it (i.e. the anti-virus part). It's best to also add in some additional anti-malware protection, such as the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/antimalware/
When installing it take care to remove the tick alongside the option to take a free trial of the Premium version. Also note that the free version doesn't 'run in the background'; you need to actively run it every so often - I suggest once per fortnight - or whenever you suspect that malware might have found its way onto your computer.