Put the FSX DVD in the drive, when it starts to run, close it.
Now go to my computer and right click on the drive and choose explore.
If it presents you with the contents of the disc, look for the FSX.exe and copy it to a temp folder.
Now go and create a back up of the FSX.exe that is in your main FSX directory and put it somewhere safe.
When that is done, move the FSX.exe that you copied from the disc into your main FSX directory and let it overwrite the one that is there.
Then try your Accel install.
If that doesn't work, then replace the new FSX.exe with your back up to put it back to the way it was.
If it does work, it should just overwrite the sp1&2 files with the ones on the Accel disc.
Did you try this? :-?