In old computers you may find different locks and keys for them. Sometimes you won't have a key and you will need to turn the lock. In many cases it is still possible by using tools not being a key (method called "picking the lock") or using a different key.
If you plan to pick a lock, remember that you may not pick it second time, so if you need to turn the "key" off and on once it's better to short wires (of the lock is electronic).

First of all, small, circular locks (tubular locks) popular in PC cases and hard drive racks. Many times the only difference on key is the inward radius of a hole e.g. thickness of the tube of which the key is made. So many times you will wind that one or two keys open all locks you find.

Large circular locks, used to protect. e.g. some IBM computers casings, are more difficult. They need unique keys. The method to pick them is to apply rotating force while gently pressing 4 or 6 protrusions until they let go. This method requires usually some tool, but if you have luck they may be picked using a ball pen tube or even cardboard tube, if it fits tight on an inner bolt providing enough rotating force.

Locks for normal keys, sometimes called "Wertheim keys" or "pin tumbler locks", are easier to pick. You need two tools: One has to fit in a keyhole and apply rotating force (it is called a "wrench"), while the second has to operate inside the keyhole and press the bolts (it is called a "pick"). I usually use a flat-blade screwdriver as wrench and a small needle-like screwdriver as a pick. So to pick the lock you need to apply torque in specified direction, then carefully press the bolts using a pick. Do it many times pressing bolts in different order. The bolts are not exactly in one row, so one bolt pressed will "lock" making the lock rotate an unnoticeable, but significant angle. Then the second bolt will be ready to press and "lock" so do the same thing with bolts again. As a result, bolts will be aligned like there is a key allowing to turn the lock one time.
sometimes there are no bolts, but metal sheet parts (it is called "wafer tumbler lock"). These locks need torque applied on the very beginning of the keyhole, so don't go deep with your wrench. The best method here is to insert a pick and remove it quickly gently pushing every part until they'll lock.
Remember that after such rotating the bolts, not held by key, may fall into the keyhole. This makes the lock almost impossible to pick again. So if you pick a lock in an old computer, do it carefully and only if you really need it.
The exception here is a lock in these plastic floppy drive cases, many times key for one lock fits to all other locks.

Source: Wikipedia