I have been having this issue for 4 or 5 days and I have tried
uninstalling/changing antivirus
disabling superfetch,prefetch,pagefile,windows search
updating BIOS
and bunch of other things that I don't remember.
The first thing I need to know is your specs. This is just to make sure that you have enough memory.
Second do you have a lot of programs on your hd? If so that is the reason for a slow boot time. Another reason is lots of startup programs.
The black screen is caused most likely be a loose connection with the screen and your motherboard on your laptop. I would suggest to go to a repair shop so that you don't break anything or void your warranty
I dont think Windows 8.1 was meant for Pentium 133 PC with 16mb of RAM.. Maybe you should reinstall Windows 95 or at least update to a PC within the last 10 years.