1. For the registry fix, it should either fix the problem or do nothing, but not affect your computer so don't worry, I've tried that fix before without difficulty! But make a system restore point if you feel worried so that you can restore if you really need to.
2. Here's the link to klite codec pack:
http://www.codecguide.com/. Once you install it, most of your common video files should be playing with no difficulty as most of the required audio/video stuff that windows media player or media player classic needs to read and play files should be installed.
Also, do check google if you don't know something like what klite codec pack was!
Yes, they were two separate things. You said open with option was missing and your flv files can't play. I provided solutions that worked for me, but I'm not at your PC and all PC's are different even if they're clones but should be working fine!
3. You don't have to open PP viewer first IF you got the power point files set to open with PP viewer as default (aka, file association is PP Viewer)