I've looked, but I don't see any way to know the exact moment that the OS is first looked at. It happens very soon after the bios checks for devices and passes the boot process off to the OS.
If you have a Hard Drive light on your computer, then the bios is trying to load the OS as soon as you see that light flashing after you hit the Power ON switch.
I'm concerned by the term "restart" being used in the OP's original post. I've seen RE-Starts act all sorts of FUNNY.
While a COLD start, acts completely different.
I've seen many computers HANG on a Restart, where I'd have to do a full
Power OFF to reset them and then a
COLD boot would come up quickly and normally. So now I try not to ever allow my PC to "Re-Start". But I'll shut it down completely and RE-Boot it.
JFYI, there is a setting in the Boot.ini, accessible in MSCONFIG, where you can change the default Boot Delay, from 30 seconds to just 3 sec's and shorten your boot time by 27 sec's.
The entire "
speed up system start" text file has been around the internet for several years and it still works.
Shorten the Boot Time in XP, Vista & Windows 7 & 8, 8.1
Go to the start button, choose run, then type msconfig and press Ok.
On the system configuration window, choose the "Boot.INI" tab.
Check “No Gui Boot”, then lower the timeout to a more manageable time.
I choose 3 seconds instead of 30. (windows won’t accept a lower number than 3)
Next choose advanced options.
This is where you can choose how many processors you have.
Most modern PC's are duo core (2 processors) with some quad core (4 processors)
or even six core....So select the number of cores you actually have then choose OK.
The Windows default is only 1 core.
Now choose apply and OK, reboot and you should see a marked decrease in boot time,
And an Increase in Run-Time efficiency.
I tried that on my own PC and it seems to Work, so I now share it with all my Customer's PC's too.
On my own PC I've also found that if I have extra hard drives, flash drives or a CD in the CD drive, my boot time will be significantly longer. Apparently, Windows will read and record the directory on every drive, during the early stages of System Boot.
So limiting the number of drives and files that Windows has to log in, can only serve to help the boot time.
(clean out all your junk files)
Just a thought,
Happy Computing!
TechnoMage