This 2010 15" MacBook Pro was running sluggishly and constantly asking for and rejecting the admin password. Turned out it was booting in "safe mode", thanks to one of the keyboard's shift keys being stuck and continually sending an 'on' message.
As it was running an older operating system, I did a clean upgrade to El Capitan to fix the keychain (password) issue. Quite a bit of fun was had working around the shift key always being on, to stop it from booting into safe mode. No amount of massaging the offending key brought it back to life, so it was time for a full keyboard replacement.
Everything comes out of the computer, due to the common syndrome known as #alwaysatthebottom. Battery, fans, optical drive, logic board. Everything that is but the trackpad and hard drive, although I took that out too and replaced it with an SSD. The backlighting panel is hard to get off without damage – it tears easily – so it's easiest just to replace it with a new one. Sixty-seven PH000 screws later, and the keyboard is out.
Putting the sixty-seven PH000 screws back in was the trickiest as they were so small and had a tendency to jump around like fleas. You don't want to lose any of those inside the computer! All back together and the computer was running better than ever before: fresh OS, solid state drive, and extra RAM.