What actually is overclocking

Reference Wikipedia:
"Overclocking is the process of making a computer or component operate faster than the clock frequency specified by the manufacturer by modifying system parameters (hence the name "overclocking"). Operating voltages may also be changed (increased), which can increase the speed at which operation remains stable. Most overclocking techniques increase power consumption, generating more heat, which must be dispersed if the chip is to remain operational."

What is the make and model of the computer you are trying to overclock? You say it's an old PC, but how old is it? If you have never overclocked anything before, then you really need to study (read) up on it before attempting to do it. Not trying to discourage you from trying, but if you do something wrong, you could cause damage to the computer.
 
I have Intel (R) pentium (R) 4 and motherboard is D101GGC
I know from experience that board will support the Pentium D 815 and 820 CPU's. That is the same board that comes in an Emachine W3502 with Celeron D 352 3.2GHz, which I owned in the past. It will support the Pentium D 800 Series but not the 900 series. The best I can remember the BIOS is locked so there are no over-clocking options.
  • Pentium D 815 runs on a 533 front side bus
  • Pentium D 820 runs on a 800 front side bus
You can pick up a Pentium D 820 off eBay for $5. That is a dual core that would blow the doors off of any over-clock, you could achieve with any Pentium 4. All in all that is an old machine and I wouldn't spend much more than $5 on it.
 
Basically making something go faster than it is supposed to go. For example, getting a go-kart and attaching rockets to it - there is a higher chance of it crashing and/or falling apart, but its going faster :)
 
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