stiley ... mine are on constantly .... they slow down and speed up ... but they never go off.
Ksanders - sounds like your 12 v feed might have failed on the mobo connector.
you ideally need one of these
http://www.maplin.co.uk/Module.aspx?ModuleNo=4 8647&source=2001&doy=search
but for a 1 off it's expensive... for a couple of quid more ... you could get a new PSU.
trouble is ... it might not be the PSU
do you have a meter?
what we want is the fan on the processor to come on.
logically
the fans feed from the 12v rail ...
the green light ... from the 5 or 3.3 v rail.
the processor could be coming on ... but because the fan isn't turning it's going critical and cutting off immediately
which is why the green light is OK but nothing else.
to complicate things I've had similar faults where it's been the sound card ... and the graphics holding the voltage down
you could try pulling all the cards CD/DVD/hdd and case fans off the mobo except the processor and fan and see if it spins ... if it does ... reconnect 1 at a time
hdd > graphics > etc.