Win 8.1 corrupted?

I'm having a major problem with my win 8.1 install. Last night I couldn't close windows (When I chose to put it in sleep or shut down it just shows welcome loading screen a half second later) so I decided to force shut down. Now when I turn on my PC the boot up is extremely slow mostly stuck at the welcome loading screen and when my wallpaper's gone and when I try to open up any apps it says NSI.dll is missing. I think my windows install might be corrupted. What do you guys think? And any suggestions on how to fix it? Thanks in advance for the help

do a startup repair

I've tried it and it didn't work :( I guess I'll have to do a fresh install of windows when I have some time