The reason why I just couldn't do any of what I wanted to do in my previous computers is cause they were not powerful enough.. and the programs didn't exists for them. I do 3D animation and my Computer now can do everything I want. Can a computer last 20 years. Or is it inescapable to buy a computer every 5 years

Well, chances are technology will advance, and you will want to do more with your computer. Especially with 3D. 3D is advancing every day and becoming more realistic. And that higher level of realism demands more power from your hardware. So in short, as long as you don't plan to use any new software, and barring any unforeseen faults, the machine should last you quite some time.

However, having said that, I should let you know that after 5 years, you can usually get several times the machine you have for only a few hundred dollars.

Computers are often outdated, before 5 years, esp if you are doing more advanced work on them

I have a 8 year old lap top , that I just replaced this week, but will still leave the lap top in my studio for checking email,
But my new netbook, is twice as powerful as my larger old laptop, and it is just a entry level that is most likely out dated before I bought it

Well it really depends what computer you buy and what you want to do on it. If you are working on animation then you are constantly upgrading your software every couple of years which would require higher specs but if somebody was just using it for web browsing or small tasks then yes I think if the computer doesn't break then it can last 20 years. If you buy a high end computer with loads of RAM and a fast CPU etc then it will last longer but are you willing to spend the amount that they are asking for or is it cheaper to upgrade every 5 years.