I've just put together a system using an ASUS A8N-Sli Deluxe motherboard. When turned on, the POST gives a single short beep, and then the ASUS splash screen is displayed, with the "Press Del to enter setup" message. However, pressing del does nothing most of the time, but after a few reboots pressing del will change the message to "entering setup", when the computer just sits there and does nothing.
I've tried running it with just a keyboard, one stick of RAM, the CPU and the graphics card, with the same result. I've also tried 3 different keyboards.
It seems to me that the motherboard is just broken. Anything else I could try?