cpu help

the cpu in my old pc is dead, got a new one from ebay this is the second time ive tried this and its doing the same as before which is the cpu fan wont spin when i start up.

the only differance i can see is the original cpu (which is dead but the fan will still spin when its installed ) ia an Athlon and the other one im trying is a Sempron
should this make a differance what would the reason for the fan bot spinning?

the old one
Athlon XP 2600+ (2133 MHz)

new one
Sempron 1.75GHz 2500+

on the Athlon it says front bus speed 266Mhz
and the Sempron is FSB 333Mhz

would that make it incompatable?

its the cpu thats dead but when the dead cpu is in the pc the fan still starts up so i know thats still working, but when i put this other cpu in and start the pc up the heatsink fan wont start, so i shut down the pc quick incase the new cpu burns out.

if i put the old cpu back in the heatsink fan starts up again.

http://support.packardbell.com/uk/item/ ... 120179#CPU

thats the bugger