Vista - UnauthorizedAccessException using XpsDocumentWriter

One of the features of the WPF app I'm developing is to allow the user to
convert a file of choice to an XPS file and write the new XPS file to the
(XP) computer file system.
I keep getting an UnauthorizedAccessException when writing the file,
although to my understanding my app has full trust. In the VS Application
Security I included FileIOPermission and even signed the assembly with a
strong name:

Public Function ImportDocument(ByVal pImportFile As FileInfo) As Boolean
Dim tempPathString As String
tempPathString = My.Computer.FileSystem.CurrentDirectory
Dim tempFullFileName As String = tempPathString & "\" & pImportFile.Name
Dim tempFileName As String =
System.IO.Path.GetFileNameWithoutExtension(tempFullFileName)
tempFullFileName = tempFileName & ".xps"
Dim xpsFile As XpsDocument = New XpsDocument(tempFullFileName,
FileAccess.ReadWrite)
Dim xpdw As XpsDocumentWriter =
XpsDocument.CreateXpsDocumentWriter(xpsFile)

Try
xpdw.Write(tempFullFileName)
Catch ex As Exception
MessageBox.Show(ex.ToString)
End Try

Any information/suggestions will be appreciated!