Here's a guide on how to install Ubuntu on a USB stick : https://help.ubuntu.com/community/Installation/FromUSBStick
No offnse, but if you can't find a guide by yourself on how to do something very simple and common - when there are already dozens of guides on this - installing Ubuntu might not be the best idea, especially for a family computer - unless you want to dual-boot, but that won't get rid of your Windows problems. I've never used the Ubuntu distro, but I've played with several distros and you need to do a lot of searching sometimes to do very simple things. That might not be the case with Ubuntu since it's more "mainstream" and popular than some other distros, so I might be wrong here.
In any cases, run a Malwarebytes scan first, and you can always do a clean install of Windows to get rid of everything and start fresh.