There's no getting round the fallibility of computers: the motherboard might die, the power supply could become beyond rescue or the whole thing might just become ridden with viruses and slow to a crawl.  If you decide to buy a replacement computer then the issue of data backup from the old machine to the new one is very important.Use these 5 steps if you find yourself in this situation:

1) Once you have unplugged the cables and peripherals from your old computer you will then be ready to remove the hard drive from the faulty computer.Your computer's handbook should show you the correct way to open the case.Many older machines will have a case that will require unscrewing to gain access, whereas others will have latches that simply have to be 'pushed' open.

2)  Now you have the components exposed you should be especially wary of causing damage via static electricity, as anyone well versed in ict facts will tell you.In order to de-static yourself touch something metal before you go near your computer.Repeat this step every time you step away from your machine.

3) Next you should locate your hard drive.This should be simple enough as it should be located at the front of the computer in a slot near the CD drive.Again, in older computers it may well be attached by screws.

4) Disconnect the hard drive PSU and data cable by gently pulling them out.Unscrew the hard drive if you need to and then remove it. 

5)  You should then insert the hard drive into the external hard drive enclosure.  Plug the data cable (USB) into an available USB port on the new working machine.The working computer should automatically recognise this as an external drive and assign it a letter as appropriate.  You will then be able to gain access to the data on the old hard drive.

If you are looking for for more sophisticated data backup solutions then you may want to approach a specialist provider.