>I need to change the logon/password of an established Windows Service;
Check the docs for ChangeServiceConfig()
http://msdn2.microsoft.com/en-us/library/ms681987.aspx
> Can I do this with C++?
You can do it with any language which can load a DLL, call a function within
it and pass it the address of a structure aka record aka user defined type.
Regards,
Will