Any Computer Techs Out There? 10 points?

I have a Dell Dimension 3000 with a P4 2.8 chip which in warm weather runs hot. It is fine in the cold of winter, but it races like an airplane ready to take off in the summer. My question is, can get I the junk fan (with a green shroud) replaced with a better cooling fan. I heard that Dell made their machines so that anything other than original parts will not fit or work on them. If it can be replaced, does anyone have a suggestion or model number on what fan is best to quiet this machine down. Thanks