It's actually not really important if you service runs in interactive mode or not. You always can specify window station and a desktop while creating a new process. There is a special parameter in StartupInfo structure for this.
You also can use the Developex free tool DevxExec to run a process from the windows service under another user.

DevxExec.exe /user: Administrator /password:qwerty cmd.exe

This command automatically grand to user Administrator permissions on the appropriate window station and the desktop and run a command shell.

Let's see the following scenario. You have a Windows service running. User Marcus has logged to the computer. And you want yours service to start the application that will be shown to Marcus on his current desktop. In other words, you need to run a new process in another session.

Again you can use DevxExec. It has a special parameter /sessionid to specify the session you what to run a new process in. For example this command could start a command shell for a logged in user under Administrator.
DevxExec.exe /user: Marcus /password:qwerty /sessionid:1 cmd.exe

• BatMonkey SendTo Module 1.06
  Simple, free time saving utility. Runs from your send to menu. Open a DOS prompt in the current folder. Send a file or directory to the command line. Set an environment variable...

• WinOne - Super Command Shell for Windows 7.4
  WinOne is a Command Language Interpreter, similar in concept to the shell CMD

• Reboot Buddy 2.1
  Reboot Buddy 2.1 automatically reboots the PC at the set time or on command. The application allows users to set multiple reboot times or reboot on Command, Reboot Buddy 2.1 allows user to Reboot, Logoff, Shutdown Lock workstation and also has a...

• Take Command/LE 11.0
  Take Command/LE offers a new approach to working in Windows, bringing users the power of the command line and the ease of use of the Windows interface

• Take Command 12.10
  Take Command offers a new approach to working in Windows, bringing users the power of the command line and the ease of use of the Windows interface. Take Command supports existing CMD.EXE commands and batch files, and adds thousands of new features, giving you more power and flexibility than you ever imagined. And CMD and PowerShell users will...

• Take Command 6.01
  Take Command offers a new approach to working in Windows, bringing users the power of the command line and the ease of use of the Windows interface

• 4NT 7.00
  If you are frustrated by the limitations of the standard Windows command prompt, then 4NT is the one command line tool you truly need. By adding thousands of features that Microsoft omitted, 4NT makes the command line easy to use, giving you more...

• TCC/LE 13.01
  If you are frustrated by the limitations of the Windows command prompt, TCC/LE is the one tool you truly need