My windows 8.1 freezes after I log in. But this always only happens on my first boot, so lets say I'm away from the computer for an hour or so and turn it on. I will log in and then about 20-30 seconds in it will freeze and I have to reboot it using the reset button on the case. Then after I log in it won't freeze.
theres been some issues with windows 8.1 freezing up for some people including me. it's most likely an older driver that's not liking the new OS update all your drivers and check for programs on startup that don't run on windows 8.1 or have no support for it.
update ethernet drivers, audio drivers, gpu drivers delete the old ones.
if this doesn't work flash your bios to the most recent version, although be careful doing this. Use a fat32 flash drive and get the drivers from your motherboards manufacturer.
I did all these things and I am no longer freezing. I also changed computer cases though and cleaned my hardware as they were very dusty.
My bios was like 2 years old and had no improved support for windows 8 on the older bios. the new bios added more support and stability for windows 8 and 8.1