When does a computer die ?
It all depends on what your definition of dying is. But basically it would be when the computer does not work any more and cannot be used. Some times it will not be able to start, reboot, or will be so slow that it is impossible to work. There are several explanations for such events. If your computer refuses to start / reboot then it most probably is hardware failure. One of the key components of your machine must have broken or does not work correctly anymore. This is most frequently due to old age, this can also happen if the computer has had several shocks, or been exposed to water. Or it could be caused by a virus, some viruses destroy or change the basic computer settings and then the computer does not work properly any more. You can fix this issues alone if you have a bit of computer knowledge and know the source of the problem and therefore can replace the faulty component. However if you do not know anything about computers the best way is to take if to professionals and ask them to have a look and repair it. If the computer is really old you might as well just buy a new one.
In the case that your computer has become really slow there are several possibilitys and options. It might be an old computer which you have had for over 5 years for example. It could also be that your harddrives are too full in such case you should uninstall unneeded programs. It could also be caused by one or several unwanted viruses, which works in the back ground of your system and therefore slows everything down. You should either delete the virus using an antivirus, or you can restore the harddrive and reinstal your operation system. However you will lose all your data.