I've seen quite often people writing code that parses thru XML returned by SPS Web services as strings instead of using XPath - cause the XmlDocument throws this error all the time if you dont use an XmlNameSpaceManager

"Namespace Manager or XsltContext needed. This query has a  prefix, variable, or user-defined function."

Heres a snippet that tells you how to use the XmlNameSpaceManager to use XPath with the XmlDocument.

Dim Document As New XmlDocument
Dim xml As String

xml = "<listitems xmlns:s=""uuid:BDC6E3F0-6DA3-11d1-A2A3-00AA00C14882"" xmlns:dt=""uuid:C2F41010-65B3-11d1-A29F-00AA00C14882"" xmlns:rs=""urn:schemas-microsoft-com:rowset"" xmlns:z=""#RowsetSchema"" TimeStamp=""2004-12-20T20:57:26Z"" xmlns=""http://schemas.microsoft.com/sharepoint/soap/""><rs:data ItemCount=""0""></rs:data><rs:data ItemCount=""0""></rs:data></listitems>"

'Since the XML returned by SPS contains namespace prefixes - we need to create a XmlNamespaceManager
Dim SharePointNamespacePrefix As String = "sp"
Dim SharePointNamespaceURI As String = "

Dim ListItemsNamespacePrefix As String = "z"
Dim ListItemsNamespaceURI As String = "#RowsetSchema"

Dim PictureLibrariesNamespacePrefix As String = "s"
Dim PictureLibrariesNamespaceURI As String = "uuid:BDC6E3F0-6DA3-11d1-A2A3-00AA00C14882"

Dim WebPartsNamespacePrefix As String = "dt"
Dim WebPartsNamespaceURI As String = "uuid:C2F41010-65B3-11d1-A29F-00AA00C14882"

Dim DirectoryNamespacePrefix As String = "rs"
Dim DirectoryNamespaceURI As String = "urn:schemas-microsoft-com:rowset"

'now associate with the xmlns namespaces (part of all XML nodes returned
'from SharePoint) a namespace prefix which we can then use in the queries
Dim NamespaceMngr As XmlNamespaceManager
NamespaceMngr = New XmlNamespaceManager(Document.NameTable)

NamespaceMngr.AddNamespace(SharePointNamespacePrefix, SharePointNamespaceURI)
NamespaceMngr.AddNamespace(ListItemsNamespacePrefix, ListItemsNamespaceURI)
NamespaceMngr.AddNamespace(PictureLibrariesNamespacePrefix, PictureLibrariesNamespaceURI)
NamespaceMngr.AddNamespace(WebPartsNamespacePrefix, WebPartsNamespaceURI)
NamespaceMngr.AddNamespace(DirectoryNamespacePrefix, DirectoryNamespaceURI)

'run the XPath query and return the result nodes
Dim xNodeList As XmlNodeList
xNodeList = Document.SelectNodes("//z:row[@ows_Title]", NamespaceMngr)

Mohammed Jeelani