<p>I am writing a batch file script using Windows command and want to change each occurrence of some blank space with ","  What is the simplest way to do that? </p>

Well it will depend a lot on how you are getting the data, but you may be able to finagle something with a For-Do construct. Make the field seperator be a space then build the string back in the do with a , between each token. Might I suggest a more robust scripting language? You should have VBScript on just about any relatively modern windows bow.