Most times it is not worth the effort.
As technology marches on all the components of the computer have to be upgraded, so what do you do with an old slow processor or old slow ram or an old slow dvd or small slow hard drive???
Some times you might to salvage a case and power supply. If they were quality products like antec or gateway and some others but you still should have the booklet which comes with the case which shows you where all the wires go.
I usually buy the most things to build a new PC from pricewatch.com sometimes you can get better deals from local box stores with after rebate prices or special sale coupons cheaper than on-line dealers. Things like memory, Hard drives, add video cards.
Local box stores only sell retail box items which come with all what you need for installation. The online dealers mostly sell OEM or white box or secondary brands of inferior quality. If you are careful you can snag some good products and prices online and build a nice PC for budget $.