How to escape invalid characters in XML to XML process

Hi, 
I'm receiving my input XML from an ASP page, and i get a lot of invalid for
XML characters. How can I escape/convert these invalid characters for XML
into valid characters. I need to do this because after the transformation I
post my output XML once again into a ASP page. This is the process:
 getXML from ASP --> applyXSLT --> output XML --> validateXML(use Schema)
--> postXML to ASP
 Can I do that in XSLT when I receive my input XML by matching these
characters and convert them? If yes, how? Validation is done by Schema. 
Is there any template XSLT Script that does that? Thank you for your help. 
If there aither ways of doing that, I would appreciate any pointers. 

Regards
Vijaya


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