fans operate on 12V DC
current so you would need to either to use the old PC
power supply (assuming its an older style that acutally has a power switch as many of the new ones use the motherboard to power the powersupply on/off, of course there is a way to "hot wire" your powersupply if you contact me via PM i can give you directions on how to hotwire a ATX style powersupply).
You could also use a 110V AC
--> 12V DC
transformer from say radioshack.
THey do make 110V AC
fans that are simular in size and apperance to computer fans. If you get those type then you can simply wire them in to your lighting setup so they come on when your timmer turns the lights on.