Stand by button disabled after having installed all drivers

Hi,

I have recently formatted my hard drive and re-installed windows xp from a SP2 cd.
Immediately after that, I have installed all the motherboards drivers and downloaded the windows xp SP3 and all the others updates available.

I noticed that the stand by button is disabled when I search for it via start->turn off computer.

My motherboard is a MSI 651M-L (MS-7005), with a chipset Northbridge-> SiS 651 (rev.02) and a Southbridge-> SiS 962L (rev.25).
It has a "Northwood" processor (Intel Pentium 4, 2800 MHz).

I went on the Micro-Star site, and I used the liveupdate feature. I tried to download the latest drivers from SiS too, but they are the same.
So, I successfully installed the latest AGP, VGA, IDE and audio drivers that are available.

The bios is the latest available too, and stand by function used to work perfectly before the format.

What am I missing?

Thank You.