Is the logged in user an administrator?

How (in VB.Net) do I find out if the logged in user is an administrator?

If you mean local admin, you should be able to get what you need from this sample.
Check out where I invoke IsMember.

Code:

---

    Private Function AddUserToWinGroup(ByVal szGroup As String, ByVal szUser As String) As Boolean

        Const MODULE_NAME = "AddUserToWinGroup"
        Const ADSI_PROVIDER = "WinNT://"

        Dim szPathGroup As String = ADSI_PROVIDER & m_szLocalMachine & "/" & szGroup & ",group"

        ' the username comes in looking like <domain>\<username>
        ' we need it to look like <domain>/<username> for ADSI to work
        Dim szPathUser As String = ADSI_PROVIDER & szUser.Replace("\", "/") & ""

        Try
            Dim entryGroup As New DirectoryServices.DirectoryEntry(szPathGroup)
            '            Dim entryUser As New DirectoryServices.DirectoryEntry(szPathUser)

            ' only add the user if not already a member
            If Not entryGroup.Invoke("ismember", szPathUser) Then
                entryGroup.Invoke("add", szPathUser)
            End If
        Catch
            m_szLastError = MODULE_NAME & ": " & Err.Description
            Return False
        End Try

        Return True

    End Function

---

VB Code:

Dim wp As New Security.Principal.WindowsPrincipal(Security.Principal.WindowsIdentity.GetCurrent)
        MsgBox(wp.IsInRole(Security.Principal.WindowsBuiltInRole.Administrator), , wp.Identity.Name)
        'for none built in groups just use a string (domain\group)
        MsgBox(wp.IsInRole("MHC\PS Admins"), , wp.Identity.Name)