you reinstall the operation system on to it when plugged into all your computer parts depends though think this only works on upgrading parts might be both

you can get a new one then re-install the operation system

and finally take eveything of it and format it then re-install the operation system

you cant keep the same software on to it, due to when you install your os it like configures to all your hardware i.e cpu, ram, etc. so just need to reinstall the os or else you can damager parts due to the drives are all wrong

If parts have been replaced, it's likely you will need to re-install the OS. If the motherboard or CPU is different, then you definitely need to install the OS again. You can use the HDD and not have to buy a new one, but you will still have to re-install an OS.

Well if the size is good and the connection type is the same as the motherboard (I guess SATA) there is no point in really getting a new one.

If you want to keep all of your programs, data, etc., you can use sysprep to hardware generalize the install so it can be used with a new PC. You will still need to activate the new installation because it will be used with a new motherboard, but this is the best solution.

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There are also instructions for capturing the disk image here, which is not relevant in this case.