You can buy a External hard drive and you can install a operating system, and you must have any OS installation DVD, if it is too costly to buy for you, you can also make a Bootable USB to install an Operating System. To make a Bootable USB we have variety of options like using third party software and also just using command line, it should be depends on what operating system you are trying to install. We suggest you to use rufus software on windows to create bootable USB for Windows and Linux OS's