Computer technology is changing very quickly. New Hardware parts releasing very fast and even the Software is also changing very fast. So if you want to keep up with the phase you need to upgrade your computer more often. But if you are on a very tight budget this is not possible. But if you like to play newer computer games and like to use newer versions of software you'll definitely have to have a good hardware on your computer. Because not everyone is good at computer hardware knowledge they usually buy a new one. For my point of view this is wasting money. But in some cases you have to throw away your PC and have to buy an new one. For example, I'm still using a old Pentium 4 computer with DDR1 Rams and AGP Video card. So in my case there will be no upgrade. I have to buy or assemble (Which I always do) a new one because the technology changed so much that i can't use my old parts anymore. So, how will you decide when to upgrade your PC ?
If you are not in to Gaming or any other computer related professional work actually there is a very less chance that you want to upgrade your existing PC. If you are using it to surf the internet, listening to music or to word processing surely you want have to upgrade it. But one disadvantage of using a older system is if you want to replace a hardware part on it it will be really difficult or may be the price for that part may be too high.
If you are not in to Gaming or any other computer related professional work actually there is a very less chance that you want to upgrade your existing PC. If you are using it to surf the internet, listening to music or to word processing surely you want have to upgrade it. But one disadvantage of using a older system is if you want to replace a hardware part on it it will be really difficult or may be the price for that part may be too high.