Firstly you could simplify the reading of the file a bit by using
Dim hashSHA1 As New SHA1CryptoServiceProvider
Dim objNewFile As FileStream = New FileStream("C:\TestCode\tblDetlBBB.xml", FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.ReadWrite)
Dim objFileName As FileStream = New FileStream(strOutputDecrypt, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)
Dim sArray(objFileName.Length - 1) As Byte
and remove the need for the StreamReader object.
However the problem is probably the line
msNewStream.Write(sArray, 0, sArray.Length - hashsize)
as you are not including the last 20 bytes of the original sArray, you need to allocate 20bytes more than the filesize rather than remove the last 20 bytes.