"My Computer Is Infected With A Very Deadly Virus"

Please help me

My system has been infected with a virus

I think I got it when I was trying to download something online

The virus has messed up my system and nothing works again

Keyboard doesn’t work

I bought external keyboard, it worked for a while, now the virus has corrupted the external keyboard and it doesn’t work again.

If I click on a file or folder, it opens hundred times till the system screen is covered and the system hangs.

If I wanna install a file, the virus doesn’t allow it. Hence installing a virus remover is very difficult

I have scanned the system endless times with the exiting antivirus that was on the system before and it detects nothing, meaning the virus has a strong persistence

I can’t connect to the internet, connecting to the internet has been disabled.

Mouse doesn’t work again too, I bought external

If I try to open softwares, it crashes and doesn’t open

Many other things I cannot remember

My friend are saying it may not be a virus, that maybe my system is old and I need a change.

But what can cause a system to have all these faults at once if not a virus.

Please help