Why does my computer randomly freeze up?

I don't have this problem with my laptop but only with my desktop, admittedly it is an old machine as I've had it since the middle of 2001 but I use it as a server or storage computer with lots of hard drives both external and internal connected to it. I can be doing some word processing or surfing in the internet and it just freezes up completely and you have to restart it which can be annoying, why does this happen? it seems to be in perfect working condition other than that and is pretty quick for its age.