Which fan??
Yes! of course! if it's the correct size and CFM rating.
in the case of CPU fans, usually 60mm on newer systems, but the depth of the fan can vary alot depending on the fan's power (how much air it blows) so you can fit a more powerful fan but not the otherway around. you may need differant length scews if the fan is deeper, try not to disturb the heatsink too much or you should re-apply thermal paste.
Nearly all are 12volts but check!
It is not recommended to delve into the power supply (DANGER!! HIGH VOLTAGE!) but this fan can usually be changed fairly easily, with caution, standard 80mm.
If yours is an Athlon 1 gig + or P4 1.4+ then you should already have additional case fans.
Northbridge active coolers normally 40mm can be changed for thermaltake Blue orb chipset kit.
Could do with you being more specific as I am just guessing at the mo! Martin Vote if you found this post helpful please!!