They should have some number of drivers for motherboard and more that you can reinstall but if this was my machine I'd open the EVENT VIEWER to see if there was a clue left behind about why it wouldn't sleep.
For now, try HIBERNATE as it's less twitchy on desktops.
My machine specs are AMD athalon 64x2 dual core 4600+,PS Antec
Earthwatts 500W, Mother Board Asus M2N4-SLI, Hard drive Seagate 7200 barracuda 320GB, Vista 32-bit OS, 2046 MB Ram, Video Gforce 7600GS,
Sound Creative Sound Blaster Audigy SE, Sony DVD DRU 810-A.
This system is a little over a year old and I have been having a few
problems lately. I had a power supply from Ultra that was problematic from the beginning randomly shutting down and finally wouldn't keep the system running. I replaced the power supply 2 weeks ago and problem was solved.
I have also been having trouble with my Comcast modem and my computer wasn't able to update or download certain file correctly or even stay connected to the web. I finally got them to replace the modem and problem seemed solved. I could get updates and download and my connectivity is good. For the last 2 days I haven't been able to put my system into sleep mode either automatically or manually. I read a thread in your forum that suggested changing to balance power options and to make sure that multi media is not keeping the computer awake. I followed these recommendations and I'm still having this trouble.
What happens is when I choose sleep from the start menu the monitor shuts of and the computer continues to run. Then the power supply hesitates like it's trying to turn of and then revs up again causing the NUM CAPS SCROLL lights to flash and I'm back on my desktop as if I've just it woken up. I've only noticed this for the last couple days and wonder if it could be an windows update causing the trouble. Any help would be appreciated.