My 16 month old imac just started running really REALLY slow .

My mac started running really slow acouple of weeks ago.still quite quick when i first start computer and with in a couple of minutes it is at a snail pace and gets worse and worse and then you just have to abandon it and somethimes can't even close down - can take 10 minutes to respond.also irritating delays when you type sometimes and when you tap mouse on something it might take 10 seconds then it moves .

Mac OS X 10.6.8 2.5 GHz intel core i5 HD space available 391.71 used 106.98 Amount of Ram 4GB 1333MHz DDR3.

help ? I think my wife might leave me over this or I might leave her!!!!!