Installing an OS over the existing OS

[Phil]

Thank you for reading this and I know that the subject sounds a bit vague.

I was given a Tosheba A100 some ten years ago that had Windows XP 
installed on it and it was unusable because it was too slow. I replaced 
Windows with a 32 bit OS which although slow was at least useable.

That laptop would not boot from  USB and it's DVD drive was dead. I 
bought an external DVD drive and made a bootable disc which solved the 
problem.

Moving on another ten years. My wife was given a Tosheba A100 and this 
one has Xububtu 18.04 installed on it. It is just about unusable because 
it is too slow. Like my original Tosheba A100, it will not boot from USB 
and it's DVD drive is dead and I no longer have the external DVD drive.

I was about to toss the laptop when I remembered unetbootin which I used 
to install Raspberry pi desktop version. Upon rebooting Xubuntu came up 
without an option to select unetbootin. I had expected unetbootin to 
overwrite the existing OS but I don't think that's the way it works.

What options do I have and, by the way, I have USB at the top of the 
BIOS boot options and I have tried selecting USB from the boot list 
after pressing F12?

It seems that unetbootin no longer installs an OS onto the hard drive, 
instead it's now used to make a bootable USB drive which I already have 
and the laptop won't boot from USB.

-- 
Regards,
Phil