It may sound nuts but I've always enjoyed un-installing and re-installing MSFS when neccesary. I've found FSX to be infinitly easier and quicker to get back up to full flying status than FS9 or before ever were.
The re-install is as easy as Harleyman just posted. FSX and the Acceleration disk is all you need. No need to worry about any of the SP updates as Acceleration won't install itself without making sure your okay.
Same goes for the FSX SDKs. They only install in one order and if your out of order, the installation of the Acceleration level of the FSX SDKs will prompt you to do it the correct way before it will even proceed.
The one big mistake I think folks make in the whole process is in the de-installation. I've always followed the golden rule of un-installing everything including add-ons in the reverse order that I installed them in the first place.
Uninstall (not delete)

addon scenerys & aircraft
Uninstall FSX utilities
Uninstall FSX SDKs
Uninstall FSX
I may be completely wrong, but my theory is not to leave behind any pieces of anything, either as a file, folder, icon or registry entry. If you un-install FSX first, it would seem to me that the un-install routines of any add-ons and utilites might get broken and force a manual delete. Manually deleting anything that has an actual uninstall routine is not a good thing in my book.
Once that starts happening, how can you be assured of a clean re-installation of anything if there are old loose end parts still dangling around.
FAC