Power button doesn't shut PC down, but Windows shutdown works

I have an asus p5b deluxe wifi-ap, about 3-4 years old, intel core 2 duo e6600 2.4GHz (overclocked to 3.15GHz and stable for a year), nvidia 8800GT 512Mb at stock, 4GB Corsair XMS2 DDR2 800MHz RAM

My windows Xp install is about 1 year old.

Anyway, I've noticed recently that my PC will not turn off when I hold in the momentary power switch. I normally don't do this because I do it the correct way (by shutting down the computer through the start menu or through a batch script that calls shutdown). However, I'vd had a couple programs freeze on me recently (Mass Effect 2 and STALKER: Chernobyl) and tried to just shutdown the PC using the momentary switch but it would not work (couldn't alt+tab, or ctrl+alt+del). Maybe I needed to hold it down longer, but I've never had to hold it down longer than about 5 seconds.

Anyway, so I just have to flip the PSU switch off and then back on. I booted back into Windows and then tried to turn it off the same way again, but this time it worked, although it took much longer and I felt like it took longer to actually go through the shutdown process.
if I boot the PC up, let it go through POST and then hold down the momentary switch it seems to shut down normally.

The only thing I can think of is that XP is not correctly recognizing the event, or even stranger, when certain programs freeze it just locks up the system completely.

Or, worst case scenario, my motherboard is starting to malfunction. Either the soundcard I have is malfunctioning or the motherboard as I am also getting a strange whisper that decreases and increases in volume in proportion to the volume level. I hope lots of different things aren't starting to go wrong with it, I don't know if I can afford another.

bump

How Windows treats the power button depends on a few things. Your motherboard, if it's a Hard or Soft switch, and XP power settings.

Old way, you hit the power button for a second, PC shuts down, that's a Hard switch.

Soft switch, which every new computer I know of has now, acts 2 ways. 1, fast push does nothing, hold it in for a while and it turns off the PC the "hard" way.

2, fast push starts the Windows Shutdown procedure, hold it in and it forces the shutdown the "hard" way.

Check your motherboard for BIOS and chipset driver and any power management updates. Then go into the power manager and see if you can find a way to chance power button behavior, also check in the BIOS for such settings.

Can you take off the front cover and hit the button directy? See if that shuts down every time. Maybe the case cover power button is shifted and is not reaching the power button on the case itself.

If it's in Windows, bios and power management drivers would be a good place to start.