startup

computer came up first with cmos checksum error, then computer started slow to come on and came up into bios on the cpu processor to set manually. then it would not come on at all, but the stand by light stays on when you have the power on but the system will not boot up. I have checked memory, replaced processor, and checked the hard drive , all are working perfectly. Suggestions as to why it is not coming on at all. Help me please.