iMac early 2006 (Intel) won't boot.

Bought this iMac on ebay, and it was listed as not having a hard drive. I thought it wasn't a big deal and i could just slap a hard drive in there because I met have like 9 of them laying around. I made sure it was fully formatted and everything. So I opened it up, put the HDD in and put it back together. I turn the iMac on and it chimes and the screen backlight comes on. Then after a while the CPU fan (I guess its the CPU fan, its in the lower left corner...) starts spinning very fast. I have tried resetting PRAM and SMU or whatever it is. all of this is to no luck. I don't want to make an appointment with genius as they will probably yell at me for buying it on ebay and opening it up myself. Please somebody tell me at least what is wrong with it!

PS: I don't know where the capacitors are so I cannot check...

They aren't going to yell at you. If this Mac was supposed to be in proper working order, then seek a refund from the seller, and file a fraud complaint with eBay if he fails to do so.