Some people will tell you that you need a new computer every two to three years. However this is not the case.

One of the reasons that people should upgrade to a new computer is that a better processor, or video card is needed. If you have a computer that has less that 1ghz processor you may want to upgrade. However, the biggest problem that people have is the lack of computer memory or RAM. Many people will buy computers with a small ammount of RAM.

In this day in age you need at least 1gb of RAM to live in today's world.

Follow up:

Most computers demand large amounts of computer memory not just processor speed and a good graphics card. The RAM is perhaps the most important part of your computer's performance in my opinion.

If your computer is less that 1gb then try upgrading your computer's memory first. But first make sure that the RAM is the right series and pin number before you do. If you have noticed that your hard drive runs a lot when programs are running this usually means that you have a small amount of computer memory and need to upgrade.

Lets say you don't think you need to upgrade the computer memory. Its over 1 gigabytes of RAM. It seems like the reason you would upgrade is for better processor speeds. Better processor speeds means that videos can be rendered faster, games will run faster, and software in general will run faster.

If you need to run video intensive programs or games then consider getting a good graphics card. One thing you should know for all computers is that you should never get an integrated video card. Even if you do not play games, an integrated video card will take memory from your RAM and slow down other parts of your computer. Always get non-integrated video cards. Integrated sound is okay because it will not take away as much processing speed or RAM as an integrated video card but it does speed up your computer if you have every device to run independent from the CPU and RAM. This means that graphics cards should have their own CPU called a GPU and sound cards should have a SPU.

People will consider getting a new computer if they are not satisfied with the performance of that computer. If you have tried all the steps above and have not resolved the problem then try getting a new computer. Continue to ask around about why people recommend a computer. I find that visiting Youtube and seeing reviews of different products helps me decide. However, it is best to build your own computer if you know how to.

Other people will want to get a new computer to use Vista. This reason alone is not worth getting a new computer because Vista may have compatibility issues with your old software. However, if you are going to Vista because of new software then it may be an alright idea. Personally, I would not recommend Vista to anyone.