Validating an xml file from a Windows service

I have a class that validates an xml file against an xsd schema and then returns "File valid or File not valid". The schema location is included in the xml files header. I very simply use a XmlTextReader to read the file and I pass this TextReader to an XmlValidatingReader. When I use this class in a windows or console application it works just fine. When I create an object of this class in a windows service and let it do it's job, even when I pass it a completely valid XML file it still returns "File not valid against schema". Can anybody tell me what the problem is? Thanks in advance, Johan.