I've got it!
After some testing and research I discovered the location in the registry where
msconfig stores the past enabled start value as well as the disabled date.
It consists of sub keys for each disabled service and each sub key has the following values:
SECOND as well as the short name of the disabled service who's data contains one of the following:
0 = boot
1 = system
2 = auto
3 = demand
5 = delayed-auto
All I have to do is read and write here when enabling and disabling services.
4 = disabled. That's why it is not in the list above. Only keys which are currently disabled are listed in that location. And I don't think XP has