How often do you replace your computer?

[SeattleSheri]

I've had my computer between 4-5 years and I fear it's dying a slow death. I have a Macbook Pro and a comparable replacement is > $2k, so needless to say, I'm not super jazzed about the prospect of replacing it. So my questions to you are ...

How often do you replace your computer?

Are you generally replacing because of your desire for new technology or out of necessity (hard drive failure, etc)?

Do you use a PC or a Mac?