Service Pack 1 needs to be installed on a "clean" retail copy of Flight Simulator X.

That includes no add-ons. Many 3rd party add-ons can cause crashes, and will need to be refreshed.

If you really don’t want to do this, then perhaps you can consider running FSX from the command line using

>fsx /ManualLoad

This provides a dialog for every dll or xml exe that gets loaded so you track down who is the offender if there is an issue.

However, most people don’t know how to run FSX from the command line, thus it’s safest to uninstall all add-ons.

If you have performed any file modification tweaks, you will need to uninstall or repair retail FSX.

Then install SP1.

Then start installing your add-ons 1 by 1.

Note:
Uninstalling retail FSX deletes ALL content of -
C:\Documents and Settings\All Users\Application Data\Microsoft\FSX
C:\Documents and Settings\USERNAME\Application Data\Microsoft\FSX

That is EVERY subfolder & file in that branch.

If you don't back up and save, you will loose config files, flights, logbook, and rewards.

Two files to consider saving:
Logbook.bin &
GrantedRewards.bin

See this support alert for full details.