If you decide to replace your hard drive and this hard drive is your main/only hard drive, I would first make sure that you have a CD for re-installing your operating system, typically if you bought the PC from built to spec you had no input into the build itself and would ultimately have been provided with an OEM version of your OS, you may not have a full installation disk for the OS, this could present a problem and it may be advised to simply use software to create a full image of the old harddrive and copy it on to the new one. Once you know that you can reinstall your OS. You can do the following:
- turn off PC and unplug the PC (also don't lick the power cables but this should be obvious lol)
- Open computer case
- look for the HDD
- you will see 2 cables plugged into the HDD one is a power cable, the other will be a SATA (or ATA if real old), unplug both from the hard drive (Be careful to not break the pins on the cable while unplugging, they tend to be firmly fixed into their connectors but try to be gentle)
- Look around the location of the HDD the HDD could be fastened within the case 2 ways, either screwed into the HDD bay (you will need a screw driver) or a removable (what I would call) plug'n'play drawer that sits in the drive bay (you will still have to use a screw driver to take HDD from the plug'n'play drawer.
- Once HDD has been unscrewed as mentioned above, you can move in the new HDD in its place and screw it in to the drive bay/plug'n'play drawer.
- Plug in the cables SATA and power cable into the new drive, make sure they are seated properly, once again try being gentle
- put on computer case lid/door/window
- plug in the power cables to your PC and turn on
- Since its a new HDD (and if you didn't copy over stuff (like the OS) to the new HDD) there wont be an Operating System installed, you may have to boot from CD, this can be achieved through the BIOS screen, you should be prompted for this as your PC should detect a blank disk that needs to be formatted for use.
- Once the boot from CD option is set, you will have to go through installation of the OS.
I hope this helps, if there is something missing I am sure some very good people will fill in the gaps.