After verification, it seems the class is OK. Returning negative values is applicable. I took another version of that class and obtained the same result. Here it is:

    Public Function GetCRC32(ByVal sFileName As String) As String
        Try
            Dim FS As FileStream = New FileStream(sFileName, FileMode.Open, FileAccess.Read, FileShare.Read, 8192)
            Dim CRC32Result As Integer = &HFFFFFFFF
            Dim Buffer(4096) As Byte
            Dim ReadSize As Integer = 4096
            Dim Count As Integer = FS.Read(Buffer, 0, ReadSize)
            Dim CRC32Table(256) As Integer
            Dim DWPolynomial As Integer = &HEDB88320
            Dim DWCRC As Integer
            Dim i As Integer, j As Integer, n As Integer

            'Create CRC32 Table
            For i = 0 To 255
                DWCRC = i
                For j = 8 To 1 Step -1
                    If (DWCRC And 1) Then
                        DWCRC = ((DWCRC And &HFFFFFFFE) \ 2&) And &H7FFFFFFF
                        DWCRC = DWCRC Xor DWPolynomial
                    Else
                        DWCRC = ((DWCRC And &HFFFFFFFE) \ 2&) And &H7FFFFFFF
                    End If
                Next j
                CRC32Table(i) = DWCRC
            Next i

            'Calcualting CRC32 Hash
            Do While (Count > 0)
                For i = 0 To Count - 1
                    n = (CRC32Result And &HFF) Xor Buffer(i)
                    CRC32Result = ((CRC32Result And &HFFFFFF00) \ &H100) And &HFFFFFF
                    CRC32Result = CRC32Result Xor CRC32Table(n)
                Next i
                Count = FS.Read(Buffer, 0, ReadSize)
            Loop
            Return Hex(Not (CRC32Result))
        Catch ex As Exception
            Return ""
        End Try
    End Function

At the Return line, if you remove the Hex(), you will obtain another version of the class returning the same value.