I get this question from time to time and the answer is pretty straight forward. If you can still manage to do everything you want to do on your computer with out moderate hassle then you don't need to upgrade!! Even if your computer is 10 years old you don't need to upgrade. Just remember the old adage, "if its not broke, don't fix it". Or in this case if you can operate on your computer just fine to get your tasks done then don't be pressured to buy the latest and greatest technology out there.
That being said, some times it is time to upgrade =P Getting a new computer that's right for you is another one of those things which i cant help you out with unless I know what you use your computer for to begin with. For example lets say you spend most of your time on the computer browsing websites and checking e-mail, you arn't going to want to spend money on a computer that's built for gaming and comes complete with a high-end 3d video graphics card.
If you really need to upgrade then make sure you speak to a qualified customer service rep at whatever store you are planning to buy a computer and make sure he knows what you use your computer for, and make REALLY sure that the computer he suggests for you can do everything you need. Or you can solicit the services/advice of a computer repair man.
That's it for now!