Here are some hints I come up with after reading some of your source code:

1. Even though the Print Spooler service is running under LocalSystem account, with the User Account Control (UAC) enabled, it doesn't run with all privileges. To add required privileges to the service without disabling UAC (which will enable all privileges, if I'm not mistaken), you have to edit the registry entry "HKEY_LOCAL_MACHINE\ SYSTEM\ CurrentControlSet\ Services\ Spooler\ RequiredPrivileges" or better yet, call the ChangeServiceConfig2 function.
2. In Visual Basic 6 services running under Windows Vista, to prevent "Interactive Service Detection" dialog from being displayed without disabling the Interactive Service Detection (UI0Detect) service, you have to select "Unattended Execution" option from the project's properties and recompile the project. If the option is grayed, you have to remove all forms and components (use Project > Components) from the project.
3. Before launching PDFCreator.exe as the logged-on user (i.e., calling CreateProcessAsUser) but after loading the user's profile (i.e., calling LoadUserProfile), you might want to call the CreateEnvironmentBlock function to load the user's environment variables (for some reasons, that seemed to be required in Windows Vista, else the user's document folders won't be located correctly). After successfully calling CreateEnvironmentBlock, don't forget to pass the environment variables to CreateProcessAsUser and call the DestroyEnvironmentBlock function in the clean-up section.

Good luck.