Can you install linux on a computer that has another operating system installed?
Yes, you can. It is known as dual booting. Install Windows
first, then any Linux distribution of choice - Ubuntu, Mint, Puppy,
and so on) second. When installing the Linux OS, you will be
offered either to wipe completely and use the whole hard-drive,
install Linux alongside Windows (or the OS already installed), or
Custom Install. To dual-boot, choose install alongside...