It depends on what it is, what your situation is, and what you want to achieve for an end goal and how much you want to spend to get there.
Sometimes older computers are best used for the kids and start from scratch on a new unit for you. Another option is to donate the old unit to a school or charitable institution for a tax write off.

If the components in the old computer are of high enough quality you can save hundreds on putting your new computer together. Let us know what you have and what you want, then we can make suggestions but rest assured, no matter what your needs, you will not find a better price/quality combination anywhere.