My iMac Can't Recognize Pre-Boot Commands Like CMD+S or D key to AHT

Hi everybody. I'm here begging a helpful answer. My iMac became freezing twice per week after Snow Leopard Installation. I already formatted my computer and reinstalled the Snow Leopard as I reinstalled everything again. The problem persisted and I use the Utility Disk to try to fix it. Nothing change and a tried to do Apple Hardware Test. I tried this anyway as possible, but my mac can't run apple hardware test. I Hold the D key and nothing happened. So someone told to download and try applejack software, holding Cmd+S key at "bong" sound. Nothing happened again, my imac started normally. So I realized there are just two pre-boot commands that work. hold C key to Snow Leopard Installation or hold the Option key to choose the initialization disk. Have anyone any idea what is happening to me? Thanx Anyway.