I made a topic about this around a month or so ago about when I start my HP Pavilion dv6t-Quad Custom Edition laptop up which is running Windows 8.1 64 bit with all the latest updates it will randomly freeze sometimes. By start up I mean either by waking the computer from sleep or turning the computer on from a previous shutdown. I do not use hibernate, as my Samsung SSD software has it disabled anyways, lol.
Before someone said that it could be a graphical issue since this laptop has switchable graphics (Intel 3000 and Radeon 6770M) I though that could be a possibility. Of course trying to update switchable graphics is not simply done because you need the official drivers from HP & Intel/AMD. I made a dual boot system a few days ago to test out Win 10 Tech Preview and I only could use it for about 5 mins before everything froze up, similar in the way it does now but I could use the PC for bit. So I but a windows 8.1 over the win 10 so I have two win 8.1's now. They both have the graphics drivers for both the display cards which are listed under the display section of "Device Manager". The other partition I haven't had this 'freezing' happen to once yet. So I wonder then if it could be something else...
Most commonly it happens when I wake the computer from sleep as I'm on the go a lot lately. So I'll open the lid, laptop starts up, sign in screen appears, sometimes the finger print reader will work when this issue happens sometimes I have to type my password in manual. It will load the desktop, I click on a few things and then it will freeze. Most commonly I check this by right away clicking on the start menu and then clicking the little side arrow next to shutdown and that will like load up 'half way' so it will appear semi transparent, its weird. But if it does that I no that I have to do a heard restart because its froze by that point and I can't click shutdown or anything else anyway.