Imports System.Runtime.InteropServices
Public Class Form1
    <DllImport("user32.dll")> Private Shared Function GetLastInputInfo(ByRef plii As LASTINPUTINFO) As Boolean
    End Function

    <StructLayout(LayoutKind.Sequential)>
    Structure LASTINPUTINFO
        Public cbSize As UInteger
        Public dwTime As Integer
    End Structure
    Dim lastInput = New LASTINPUTINFO()
    Dim alreadyWarned As Boolean = False
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        lastInput.cbSize = Marshal.SizeOf(lastInput)
        lastInput.dwTime = 0
    End Sub

    Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick

        GetLastInputInfo(lastInput)

        If ((Environment.TickCount - lastInput.dwTime) / 60000) >= 30 Then '>= 30 minutes
            If Not alreadyWarned Then
                alreadyWarned = True 'Do this so we don't spam a message box every second
                MsgBox("You've been idle for a while")
            End If
        Else
            alreadyWarned = False 'Reset if the system is no longer idle
        End If

    End Sub
End Class