If you have recently updated your BIOS, please clear RTC RAM (CLRTC) jumper first, and load default at the BIOS menu. Sometimes, clearing RTC RAM (CLTRC) jumper and load BIOS default can help eliminate some of unexpected phenomenons.
If this phenomenon still persists, it may be caused by VISTA OS. VISTA does not allow system to enter Standby mode, if "Allow this device to bring the computer out of standby" for an USB or PS/2 wake-supported device (such as mouse or keyboard) has been checked under "Device Manager", while the USB or PS/2 port has not switched its power mode to +5VSB that allows delivery of standby power to USB or PS/2 ports under soft OFF state. Because VISTA enables this standby option per default, and motherboards often set USB power modes to +5V at the same time, these two settings may result in conflict, hence resulting this phenomenon.
If you would like to wake your system from USB or PS/2 devices, please turn off your PC, set the appropriate USB port or PS/2 port jumper from +5V to +5VSB. Please follow the instructions in your user manual. Then your system will be able to enter and wake from standby mode as you desire.