I did my last reinstall of FSX according to an article I read in PCPilot (Nr.68 Juli/Aug2010) as the followed:
1. Install FSX, start it and starting a freeflight, shuting down FSX, Reboot PC
2. Install SP1 for FSX, start FSX and starting a freeflight, shuting down FSX, Reboot PC
3. Install Acceleration (Note PCPilot said SP2 isn`t necessary when installing Acceleration, as SP2 is included in Acc), start FSX and starting a freeflight, shuting down FSX, Reboot PC
when installing the sdk`s they said FSX sdk should be installed after the above step1 and the Acceleretion sdk after step 3.
After that I installed all my addons and sceneries.
I donĀ“t know if this is the "Right" way of installing FSX. BUt it worked well for me. With my old installation I had quite often FSX crashed in Flight. Since reinstalling it works much better.