How do I force my .NET application to run as administrator?

You'll want to modify the manifest that gets embedded in the program. This works on Visual Studio 2008 and higher: Project + Add New Item, select "Application Manifest File". Change the <requestedExecutionLevel> element to:

 <requestedExecutionLevel level="requireAdministrator" uiAccess="false" />

The user gets the UAC prompt when they start the program. Use wisely; their patience can wear out quickly.

Adding a requestedExecutionLevel element to your manifest is only half the battle; you have to remember that UAC can be turned off. If it is, you have to perform the check the old school way and put up an error dialog if the user is not administrator
(call IsInRole(WindowsBuiltInRole.Administrator) on your thread's CurrentPrincipal).

The detailed steps are as follow.

1. Add application manifest file to project
2. Change application setting to "app.manifest"
3. Update tag of "requestedExecutionLevel" to requireAdministrator.

Note that using this code you need to turn off the security settings of ClickOnce, for do this, go inside Properties -> Security -> ClickOnce Security

I implemented some code to do it manually:

using System.Security.Principal;
public bool IsUserAdministrator()
{
    bool isAdmin;
    try
    {
        WindowsIdentity user = WindowsIdentity.GetCurrent();
        WindowsPrincipal principal = new WindowsPrincipal(user);
        isAdmin = principal.IsInRole(WindowsBuiltInRole.Administrator);
    }
    catch (UnauthorizedAccessException ex)
    {
        isAdmin = false;
    }
    catch (Exception ex)
    {
        isAdmin = false;
    }
    return isAdmin;
}

You can embed a manifest file in the EXE file, which will cause Windows (7 or higher) to always run the program as an administrator.

You can find more details in Step 6: Create and Embed an Application Manifest (UAC) (MSDN).