@kdkjan
Do this:-
run regedit (Windowskey +R)
Expand the following registry key: HKEY_LOCAL_MACHINE\SYSTEM\CURRENTCONTROLSET\CONTRO L\CLASS{4D36E967-
E325-11CE-BFC1-08002BE10318}
After you find the folder, double-click Upperfilters on the right side.
Change the value from 0 to PartMgr and/or remove any other settings and then click OK.
Close Registry Editor
Start FSX/Accel and you should get the registration wizard.
(Just to make it clear. Change partmgr to PartMgr).
The next time you launch FSX, you'll have the prompt to enter the ACCELERATION product key.
Closed/Opened a few times, and all seems well.
..