Sleep Mode would only come on after Windows has been working for some time (by default it's no less than 15 minutes). The BIOS has it's own power saving options but they are meant to work along with Windows... What you may be experiencing is the monitor going into sleep mode... I have one monitor that goes into sleep mode immediately after powerup but it only does it, if it doesn't rightaway get a video signal, and quickly comes out of it as soon as it gets a signal. So, that may be what's going on with your monitor. If the computer has both onboard video and PCIe Graphics card, your monitor may be connected to a disabled video port, so try another one.
And if that doesn't work, reset the BIOS as follows:
Remove the AC Power cable...