How to protect data of VB.Net app in output file

Hello everybody

Actually i have deevloped an VB.Net application for Windows Mobile 2003, that keeps log data (String format) in a file created by stream writer. But the problem is that any body can open the on the iPAQ and on Desktop PC and change its contents because that file is easily readable.

Is there anyway in Compect Frame work , like we do in normal desktop based application by keeping data in a file generated in Random access mode to write custom data types for its protection. In this way we have two advantages first file format is proprietry and its safe

Please somebody tell me about this

Thanks