"InstallShield for Visual C++" is a limited edition of InstallShield Professional 5. It doesn't have built in support for services. You can write InstalLScript code to call the relevant Windows APIs, and there is sample code for this here on InstallSite. However this will work only with the full version, not with the limited edition, because calling Windows APIs is disabled in the limited edition.
You could look into Visual Studio Installer which is available as free download from Microsoft for Visual Studio 6 owners. I'm not sure whether this supports NT Services.
Otherwise you should consifer moving to a full featured setup authoring tool. The successor to InstallShield Professional is InstallShield DevStudio which can use InstallScript with no limitations. Until Nov 30 you can get DevStudio at an introductory offer (see
here for details and evaluation).