An aging computer that cannot quite meet the requirements for a Windows 7 upgrade can sometimes find a few more years of usefulness with a switch to the Linux operating system. Ubuntu, which makes a free and easy-to-use version of Linux, has traditionally made it easy to try out the system by posting instructions for making a compact disc that can boot up and run Linux for a test drive before installing it. But for old computers with broken disc drives, Ubuntu Linux can also be run from a USB flash drive. The Ubuntu site has detailed instructions here for making a bootable USB stick (or CD) with the current version of Ubuntu from either a Windows or Mac computer, and an online demo here.
