I have never overclocked before but I am a fast learner and very good at computers. The PC that I would like to overclock is an old HP Compaq Presario 7180. It has 16 MB of RAM, 1.2 GB HDD, Windows 95 OS, the processor has 100 MHz of speed, and a 150 watt power supply.
For old, old computers, check on the motherboard for jumper settings. You may have a selection of jumper settings for the bus speed, and one for multiplier. The total CPU frequency = multiplier X bus speed.
However, on older computers, there really isn't a lot of room to OC, count your blessings if you get 10% out of it.