VS 2008 Get SID from username in a domain

Hi

I have created the code "

  PrivateSub Button5_Click(ByVal sender As System.Object,ByVal eAs System.EventArgs)Handles Button5.Click

Dim sidstringAsString = txtUsername.Text

Dim accountnameAsString =New WindowsIdentity(sidstring).User.Value

  MsgBox(accountname)

EndSub"

But i need it to look up the information from our domain any idea how I would change it to do that?

Thank You