When I was working as a Desktop Support Technician in college, I wrote a .NET C# Dell service tag finder because I dealt with well, many Dell computers. I decided to write a little program that I could carry on my flash drive with me and all my other technical applications. What I like most about this application is the ability to quickly get to the Dell Warranty and driver information. I hope that someone else finds this useful as well, if you do let me know.

Download Dell Service Tag Finder: Source | Executable

This is was possible using Windows Management Instrumentation and if you are interested in pulling other information from your computer I recommend you read up on it. For this example, I used the Win32_BIOS Class, but you can find all the WMI Classes here.

Preview of Source:

        String dellServiceTag;

        private void Form1_Load(object sender, EventArgs e)
        {
            //Load Service Tag
            ManagementClass wmi = new ManagementClass("Win32_Bios");
            foreach (ManagementObject bios in wmi.GetInstances())
            {
                dellServiceTag = bios.Properties["Serialnumber"].Value.ToString().Trim();
            }
            Display.Text = dellServiceTag;
        }

Notes:
This application requires the .NET Framework or you may receive application errors.
The service tag is pulled from the BIOS so if you ever had your motherboard replaced and the service tag was not reset it will be blank.
The service tag is really the Serial of the machine, so other models such as HP will display the Serial, but the express code will be wrong. I will look into this when I have multiple machines to test on.