the psu luke the PSU..try another PSU...

alwasys a good idea to remove all cards when testing..

If your to use another OS.. try a BART PE .. (read Iron geeks tutorial), if it doesnt shutdown, try another PSU..if it does you will need to look at the drivers for the mobo.. windows isnt sending the KILL to the BIOS for the final shutdown..

I havent had to fault find this one for a while.. but If you USE A VOLT METER.. there is one pin that controles the PSU from the MoBo, if it dosent change then back to drivers.. but if it droops, it is a compatability issue between the mobo and PSU..try a different BRAND of PSU

You are able to measure the voltages aren't you?