Usually I don't respond to enquiries if they are urgent, because it often means that people don't take the time needed to formulate the question, gather the evidence, and study the answer carefully.
To help you diagnose a problem like this we need to know exactly what you did and exactly how it failed. That means you need to show us your XML and XSLT input (or a representative sample), you need to say how you were running it (with product/version details if appropriate), and you need to quote the actual error message, not just a precis.
The most likely thing is that one of your input XML documents is not well-formed, because of the illegal characters. XSLT processors cannot handle badly-formed input; you will need to correct the input before you start any XSLT processing. It's best to do that by correcting the program or process that generates the XML (more accurately, the non-XML) in the first place.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference