Same issues here, I have been working on my dad's 500t for a week straight now. Through trial and error I have found out for sure what the issue is. The REALTEK I2S software/drivers and windows 8.1 have some kind of failure syncing up and this seems to be preventing windows 8 from updating to 8.1. Then the problem after that is updating and reinstalling the sound drivers, every time I try it becomes unbootable. Its a good thing I have manually been creating restore points every step of the way after 8.1 finishes.
This is what I did:
1)reset windows 8 to factory specs
2)uninstall all unnecessary apps and programs (Including antivirus programs) *Restart
3)Update windows 8 all the way (*restart)
4)Update any remaining apps all the way (*restart)
5)uninstall realtek, then clean the disk out of junk (*restart)
6)Setup the clean boot and restart
7)Go to the Microsoft Store and Update to 8.1
8)After 8.1 finishes IMMEDIATELY ENABLE DISK PROTECTION AND CREATE A RESTORE POINT.
*From this point forward any progress you make and get a successful restart create a new restore point.
- Clean the disk again and undo the clean boot settings
10)Start with important Windows Updates small chunks at a time (5 or so at a time restarting and creating restore points between each)
Now I have just one problem to overcome, sound... Every time I try, even with other drivers, or from the device manager nothing works, someone needs to rewrite the driver from scratch instead of patching the old one.
In fact I don't think I have seen one instance where a 500t is currently running 8.1 successfully.