You said this free system is pretty old, how old do you think it is? If your just wanting to bring this old system back to its "just bought" condition you can probably do that. If your wanting to upgrade it to play todays fast games, you will probably be out of luck.
Things change fast in the computer world. If you purchase the newest chip out today, within 6 months it will an old chip. Still a good chip, just not the latest. Now think about how many changes have happened in 3 years and you can get an idea of how out dated your system is.
You might be able to use the hard drive, cd/dvd drive and computer case. But I am betting every thing else will have to be replaced. If this computer was made by one of the big company's like Dell, HP, Compaq, Acer, you can bet that nothing is upgradeable. They all use special parts made for them exclusively by parts manufactures. The old parts will not fit (except for the hard drives and cd/dvd drives) in any new case, and new parts ( like the motherboard) will not fit in the old case.
Let us know what you are getting. And I'm sure someone can help get you upgraded as much as possible.