I am looking for a script to accomplish the follwoing.

Open a text file on the users desktop c:\\documents and setting\username\desktop\filename.
Search the file for a string and replace it with a new string. Then save the new file.
I have been told to look at READLINE, WRITELINE AND INSTR, but I am a newbie to Kixtart and scripting so I am am at a loss. Any help will be greatly apriciated.

Sean