Unfortunately, getting it running on that old hardware is not going to be a simple "point and click" type task... Granted there are still Linux distros out there catering to "old hardware", but they usually involve an amount of customisation to run on CPUs missing certain "modern" features and instruction sets (PAE, SSE3 etc).
Without knowing the exact specs of what you're running... ie. the exact CPU/mobo model and the amount of RAM we're working with, it's difficult to make a specific recommendation on which linux distro you should try (and how to configure it).
Alternatively, the "easy but more expensive" option is to simply buy updated hardware