XMLReader versus SAXParser

I have a series of SAX2 applications that use the constructor SAXParser() to create an instance of the parser.
XMLReader reader = new SAXParser();
These applications work fine.
The O'Reilly book on Java and XML indicates that this class (SAXParser) implements the org.xml.sax.XMLReader interface (pg. 50). If I change my code to use XMLReader instead of SAXParser, I get a ClassNotFoundException: org.apache.xerces.parsers.SAXParser.
Here's the code line that changed:
XMLReader parser = XMLReaderFactory.createXMLReader(
"org.apache.xerces.parsers.SAXParser");
Can anyone explain to me why? This problem is also causing me troubles when trying to use Xalan.

I'm touching this in the hopes of someone seeing it at the top of the list. I'm really stumped on this problem.

Does it work if you use SAXParserFactory.newSAXParser() ??

Well, I got this to work by implanting the code from XMLReaderFactory into my parsing class. In particular the line:
return (XMLReader)(Class.forName(className).newInstance());
doesn't throw a class not found exception when it is within 'my' class that parses the document.
I still don't know why it wouldn't work before.
I still think I have a problem when using Xalan but we'll see.

I got it working in the manner that it should. I had to place the appropriate jar files in the lib/ext folder. Having them in the class path didn't seem to cut it.