Here is the fix for the activation error you might recieve when installing Microsoft Flight Simulator X w/ Acceleration Pak.
1) with regedit, located this entry:
HKEY_LOCAL_MACHINE\SYSTEM\CURRENTCONTROLSET\CONTROL\CLASS\{4D36E967-E325-11CE-BFC1-08002BE10318}
2) edit your UpperFilters key. For FSX and their validatin system, they
expect it only reads "PartMgr". In your case, there is great chance it shows
something more. In my case, it was reading:
GoBack2
PartMgr
SiRemFil
3) change the registry to read only "PartMgr"
4) start FSX, you should now be prompted with the activation dialog, proceed
and let FSX start, then exit FSX.
5) edit back the registry key with your previous content
6) exit regedit, and start FSX again, it should all be fine by now!