Ed,
thanks for your advice/

I use the following code in MS Office 2003/SP-2/WinXP SP-2:
Sub Moltest()

' Moltest Macro
' Macro created 02.08.2006 by IA
'Dim x, y, z As String, MyName As String
  Dim i As Integer
  Dim Response As Integer, TotalFiles As Integer

  On Error Resume Next

Folder:

  ' Prompt the user for the folder to list.
 X = InputBox(Prompt:="What folder do you want to list?" & vbCr & vbCr _
        & "For example: C:\My Documents", _
        Default:="G:\Docart")

 y = InputBox(Prompt:="What files do you want to look for?" & vbCr & vbCr _
        & "For example: *.doc", _
        Default:="*.doc; *.htm; *.txt")

 Z = InputBox(Prompt:="Insert keywords" & vbCr & vbCr _
        & "For example: Reactor And nuclear")

  If X = "" Or X = " " Or y = "" Or y = " " Or Z = "" Or Z = " " Then
     If MsgBox("Either you did not type a text correctly" _
           & vbCr & "or you clicked Cancel. Do you want to quit?" _
           & vbCr & vbCr & _
           "If you want to type once more, click No." & vbCr & _
           "If you want to quit, click Yes.", vbYesNo) = vbYes Then
        Exit Sub
     Else
        GoTo Folder
     End If
  End If

  ' Test if folder exists.
  If Dir(X, vbDirectory) = "" Then
     MsgBox "The folder does not exist. Please try again."
     GoTo Folder
  End If

  ' Search the specified folder for files
  ' and type the listing in the document.
  With Application.FileSearch
     .NewSearch
     .FileType = y
     ' Change the .FileType to the type of files you are looking for;
     ' for example, the following line finds all files:
     ' .FileType = msoFileTypeAllFiles
     .SearchSubFolders = True
   .FileName = ""
   .MatchTextExactly = False
   .TextOrProperty = Z
     .LookIn = X
     .Execute
     TotalFiles = .FoundFiles.Count
     If TotalFiles = 0 Then
        MsgBox ("There are no files in the folder!" & _
              "Please type another folder to list.")
        GoTo Folder
     End If

     ' Create a new document for the file listing.
     Application.Documents.Add
     ActiveDocument.ActiveWindow.View = wdPrintView

     ' Set tabs.
     With Selection.ParagraphFormat.TabStops
        .Add _
              Position:=InchesToPoints(3), _
              Alignment:=wdAlignTabLeft, _
              Leader:=wdTabLeaderSpaces
        .Add _
              Position:=InchesToPoints(4), _
              Alignment:=wdAlignTabLeft, _
              Leader:=wdTabLeaderSpaces
     End With

     ' Type the file list headings.
     Selection.TypeText "File Listing of the "

     With Selection.Font
        .AllCaps = True
        .Bold = True
     End With
     Selection.TypeText X
     With Selection.Font
        .AllCaps = False
        .Bold = False
     End With
     Selection.TypeText " folder!" & vbLf
     Selection.Font.Underline = wdUnderlineSingle
     Selection.TypeText vbLf & "File Name" & vbTab & "File Size" _
           & vbTab & "File Date/Time" & vbLf & vbLf
     Selection.Font.Underline = wdUnderlineNone
     For i = 1 To TotalFiles
        MyName = .FoundFiles(i)
        Selection.TypeText MyName & vbTab & FileLen(MyName) _
              & vbTab & FileDateTime(MyName) & vbLf
     Next i

     ' Type the total number of files found.
     Selection.TypeText vbLf & "Total files in folder = " & TotalFiles & _
           " files."
  End With

  MsgBox ("The number of files is " & TotalFiles & _
           " files.")

  If MsgBox("Do you want to list another folder?", vbYesNo) = vbYes Then
     GoTo Folder2
     Else
     GoTo Konets1
  End If
Folder2:

Konets1:

End Sub

If I use the first keyword, it is about 1000 files in TotalFiles, with
second it become 3000 and with third - 6000.
It looks like instead of AND code uses OR function, and I do not realize,
why it happens

Petr.