Well this ain't good, i have to say i was hoping for better things from the last we heard from your post, but now, this mobo is like a hot potato. I had a very bad feeling from all the posts i've been reading. And i wouldn't have activated win 7 yet anyway, i always leave that to the last or so day, so to be sure theres nothing wrong with the install. Let us know how you get on.
Well I normally wait to activate as long as I can, but where I was trying to get this shutdown issue fixed by changing stuff in the BIOS Windows threw a fit and said I wasn't using a Genuine Windows 7 copy which I lost all my 30 days of trial, so I tried to activate so i could continue but it failed, I then read some posts from microsoft showing steps how to reset the hardware changes to the system and that resolved the Genuine issue giving me my 30 days back.
Don't get me wrong this board is fast and great, just this shutdown issue, I have also noticed its not just Gigabyte that are having this issue but some ASUS and one or two others, something todo with the SATA controller drivers or something.
Don't forget these boards are new and will have some teething issues at the start.
They will resolve this issue, well I hope they do, I dont want to return this board and memory have to go back to my Intel DX58SO crappy but stable board with cruddy 6GB of ram where my CPU HSF hits the side of my case
As I have said I will be trying different ways to see which ones cause this shutdown issue and I will post on here the results.
First try of something will be to press that button of the MB (CMOS reset) to see what that does
*** Update ***
Good news so far

After shutting down windows and then pressed the power button for 4 seconds the force off the PC.
I then pressed the clear CMOS button at the back of the case for say 4 seconds (I was very paranoid about doing this due to some people have startup issues after doing this).
Anyway I crossed me fingers and then turned on me PC, went into my bios and had to set my CPU's multiplier back to x20 to give me my 2.667GHz CPU, I als left my memory to use only SPD (NOT XMP) so it was running my 1600MHz mem at only 1066MHz or 1067MHz.
I then enabled all SATA's Intels SATA to RAID (duno why i set it to this), Marvell 6G SATA to AHCI (for advance SATA, NCQ etc), Gigabytes SATA to AHCI (i think), also turned off the onbard audio, I then saved the bios and booted into windows, logged in and then told Windows to shut down and it shutdown which I then crossed my fingers again and it stayed off.
I have just changed the memory to run it at it correct speed of 1600MHz and booted into windows to post this.
I will update this text to report the results.
Alls looking good so far
*** Update ***
Above changes was ok.
Just disabled the Full Screen LOGO Show (I like to see my post screen

)
Just disabled Gigabytes SATA Controller (Had nothing connected to it)
I will get back when I have tried these changes.
Oh btw I have all 4 SATA Controllers drivers installed (Intel, Marvel, Gigabyte and JMicron for eSATA), once I have got the BIOS as I want it I will start to remove the Drivers for the the disabled devices and get back to report the results.
*** Update ***
Ok I have just changed Intels Controller to AHCI and shutdown a few times and all seems to work fine.
Soooo, I think this may of been a coruption in the CMOS when I updated the BIOS.
So I think every time you flash the BIOS you should turn off the PC and then clear the CMOS (make sure it has the blue light on, not too sure if it needs to be lit, but mine was and it cleared fine)
Ok, now to uninstall all the non needed drivers etc.