old keg

What kind of keg is it? If it's a standard quarter- or half-barrel type keg, you probably won't be able to put your own brew in it. You would probably need specialized fittings above what's available in a homebrew supply shop. They'd be very hard to clean at home. And, unless you are up to making really large batches, it's probably way to big for kegging homebrew.

If it's a 5 gallon cornelius keg, then, yes, you can easily use it. All you would need are some quick disconnect fittings, tubing and a CO2 tank.

Although I haven't done this yet, you could turn a 1/4- or 1/2 barrel keg into a boiler by cuting off the top and getting a cajun cooker to handle the size of the keg.