Why is my iMac slow and crashes

Every since the latest up date to mountain lion my iMac is slow, crashes. I have particular problems with aperture 3. I can't have two programs going at once - if I do I get the spinning wheel of death. If I get the spinning wheel it will spin for hours if I let it. I'm force to try "force quite" but more often I simply have to turn the iMac off at the back. Help please.