Hi ,

I have found a complete solution for my problem on following link.You need to include partner WSDL to use this.I have a mdified version of this using asp file upload control.Feel free to contact me if you want that code 

http://community.salesforce.com/sforce/board/message?board.id=NET_development&thread.id=1876&view=by_date_ascending&page=1

You can refer following code snippet which is nothing but the modified version of the code on above link.Here Resume_upload is asp file upload server control.The code is written in VB.NET

If Resume_upload.FileName <> "" Then

            Dim xmlDoc As System.Xml.XmlDocument = New System.Xml.XmlDocument
            Dim any(3) As System.Xml.XmlElement

            'We can do a similar thing for an attachment
            Dim attach As sForcePartner.sObject = New sForcePartner.sObject
            attach.type = "Attachment"
            attach.Any = any

            attach.Any(0) = xmlDoc.CreateElement("Name")
            attach.Any(0).InnerText = Resume_upload.FileName
            attach.Any(1) = xmlDoc.CreateElement("IsPrivate")
            attach.Any(1).InnerText = "false"
            attach.Any(2) = xmlDoc.CreateElement("ParentId")
            attach.Any(2).InnerText = strRecordId
            attach.Any(3) = xmlDoc.CreateElement("Body")
            attach.Any(3).InnerText = Convert.ToBase64String(Resume_upload.FileBytes)

            Dim sr As sForcePartner.SaveResult = binding.create(New sForcePartner.sObject() {attach})(0)
            If (sr.success) Then
               'success

            Else
             'error

            End If
        Else
            'No attachment

        End If