I usually use the stream editor "sed" for this type of stuff.

If you want to create XML with .NET then Xml(Text)Writer is the tool and
its methods will do all escaping necessary for you e.g.

    XmlTextWriter xmlWriter = new XmlTextWriter(Console.Out);
    xmlWriter.Formatting = Formatting.Indented;
    xmlWriter.WriteStartDocument();
    xmlWriter.WriteStartElement("root");
    xmlWriter.WriteElementString("text", "Kibo & Xibo");
    xmlWriter.WriteElementString("comparison", "a < b");
    xmlWriter.WriteEndDocument();
    xmlWriter.Close();

will write

<root>
  <text>Kibo &amp; Xibo</text>
  <comparison>a &lt; b</comparison>
</root>

so everything is escaped as necessary.

If that does not help then tell exactly which kind of "invalid"
characters you have or maybe check first whether some of the other methods
<http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlr
fSystemXmlXmlTextWriterMethodsTopic.asp>
Xml(Text)Writer provides like WriteBase64 or WriteBinHex do what you need.