Someone has installed Linux, or at least the bootloader. It is impossible for it to install itself. Someone booted to a CD or USB, and at least installed grub - possibly a botched Linux install.
To fix this (with a Linux installation) then just boot and keep pressing F12 over and over again - as soon as you turn the power on start pressing F12 over and over again. Don't just press and hold it, press it over and over again.
The above presumes that you already have a useful USB with valid installation media on it. If you don't, there are tons of tutorials that explain how. Basically, you download the .iso of your choice and either *write* (not copy) it to CD/DVD as an image or you "write" it to USB with a tool like Balena Etcher.