Delete a file that is in use by another process with VB.net?

Well, that's not easy.
For a file to be deletable, there has to be no open handles to it.
You can either terminate or kill the processes helding those handles, or close the handle itself. Be careful, the other process won't be aware of this. You might crash important applications - but could be better than killing it anyway. Still, that's your concern :)

So, assuming that you can determine which file is locked, you have a complete description here, how to gather all handles, how to filter the file handles and to find the file you are looking for. When you have the handle, you can close it.
See: http://forum.sysinternals.com/howto-enumerate-handles_topic18892.html[^]

It is not VB.NET. You will probably need a lot of P/Invoke, so this might be helpful: NtQuerySystemInformation API Works on 32bit but not on 64bits?[^]

1) You should Try to find the default program(executable) of the file:

Dim key As Microsoft.Win32.RegistryKey = Microsoft.Win32.Registry.ClassesRoot
    Dim secondKey As String
    Dim rtn As String = ""

    If Not ext.StartsWith(".") Then
        ext = "." & ext
    End If

    key = key.OpenSubKey(ext)

    If key IsNot Nothing Then
        secondKey = key.GetValue("").ToString()

        If secondKey <> "" Then
            key = Microsoft.Win32.Registry.ClassesRoot.OpenSubKey(secondKey & "ShellOpenCommand")
            rtn = key.GetValue("").ToString()
            rtn = System.Text.RegularExpressions.Regex.Match(rtn, """(.*?)""").Groups(0).Value
        End If
    End If

    Return rtn
End Function

2) Kill the default process of the file
3) Continue...