Check your Ram, and check your bios...
On the memory side, ASUS P5B has four DDR2-DIMM sockets, supporting up to 8 GB officially up to DDR2-800, however this motherboard supports DDR2-1066/PC2-8500 memories (we installed four DDR2-1066 modules and they worked just fine at 1,066 MHz). On this motherboard sockets 1 and 3 are yellow and sockets 2 and 4 are black. Configuring DDR2 dual channel on this motherboard is pretty easy: just install each module on a socket with the same color.
http://www.hardwaresecrets.com/article/408