Word won't do it on its own, but it's not hard to do.

1. Save a copy of your document.
2. Delete all punctuation, numbers, and any other matter you're not
interested in.
3. Replace all spaces with paragraphs.
4. Copy the list to Excel.
5. Use the sort and count functions to reduce the list to unique words and
the corresponding counts.

Sub WordFrequency()

        Dim SingleWord As String           'Raw word pulled from doc
        Const maxwords = 9000              'Maximum unique words allowed
        Dim Words(maxwords) As String      'Array to hold unique words
        Dim Freq(maxwords) As Integer      'Frequency counter for Unique
Words
        Dim WordNum As Integer             'Number of unique words
        Dim ByFreq As Boolean              'Flag for sorting order
        Dim ttlwds As Long                 'Total words in the document
        Dim Excludes As String             'Words to be excluded
        Dim Found As Boolean               'Temporary flag
        Dim j, k, l, Temp As Integer       'Temporary variables
        Dim tword As String                '

        ' Set up excluded words
'         Excludes =
"[the][a][of][is][to][for][this][that][by][be][and][are]"
        Excludes = ""
        Excludes = InputBox$("Enter words that you wish to exclude,
surrounding each word with [ ].", "Excluded Words", "")
'        Excludes = Excludes & InputBox$("The following words are excluded:
" & Excludes & ". Enter words that you wish to exclude, surrounding each
word with [ ].", "Excluded Words", "")
        ' Find out how to sort
        ByFreq = True
        Ans = InputBox$("Sort by WORD or by FREQ?", "Sort order", "FREQ")
        If Ans = "" Then End
        If UCase(Ans) = "WORD" Then
            ByFreq = False
        End If

        Selection.HomeKey Unit:=wdStory
        System.Cursor = wdCursorWait
        WordNum = 0
        ttlwds = ActiveDocument.Words.Count
        Totalwords = ActiveDocument.Words.Count

        ' Control the repeat
        For Each aword In ActiveDocument.Words
            SingleWord = Trim(aword)
            If SingleWord < "A" Or SingleWord > "z" Then SingleWord = ""
'Out of range?
            If InStr(Excludes, "[" & SingleWord & "]") Then SingleWord = ""
'On exclude list?
            If Len(SingleWord) > 0 Then
                Found = False
                For j = 1 To WordNum
                    If Words(j) = SingleWord Then
                        Freq(j) = Freq(j) + 1
                        Found = True
                        Exit For
                    End If
                Next j
                If Not Found Then
                    WordNum = WordNum + 1
                    Words(WordNum) = SingleWord
                    Freq(WordNum) = 1
                End If
                If WordNum > maxwords - 1 Then
                    j = MsgBox("The maximum array size has been exceeded.
Increase maxwords.", vbOKOnly)
                    Exit For
                End If
            End If
            ttlwds = ttlwds - 1
            StatusBar = "Remaining: " & ttlwds & "     Unique: " & WordNum
        Next aword

        ' Now sort it into word order
        For j = 1 To WordNum - 1
            k = j
            For l = j + 1 To WordNum
                If (Not ByFreq And Words(l) < Words(k)) Or (ByFreq And
Freq(l) > Freq(k)) Then k = l
            Next l
            If k <> j Then
                tword = Words(j)
                Words(j) = Words(k)
                Words(k) = tword
                Temp = Freq(j)
                Freq(j) = Freq(k)
                Freq(k) = Temp
            End If
            StatusBar = "Sorting: " & WordNum - j
        Next j

        ' Now write out the results
        tmpName = ActiveDocument.AttachedTemplate.FullName
        Documents.Add Template:=tmpName, NewTemplate:=False
        Selection.ParagraphFormat.TabStops.ClearAll
        With Selection
            For j = 1 To WordNum
                .TypeText Text:=Words(j) & vbTab & Trim(Str(Freq(j))) &
vbCrLf
            Next j
        End With
        ActiveDocument.Range.Select
        Selection.ConvertToTable
        Selection.Collapse wdCollapseStart
        ActiveDocument.Tables(1).Rows.Add BeforeRow:=Selection.Rows(1)
        ActiveDocument.Tables(1).Cell(1, 1).Range.InsertBefore "Word"
        ActiveDocument.Tables(1).Cell(1, 2).Range.InsertBefore
"Occurrences"
        ActiveDocument.Tables(1).Range.ParagraphFormat.Alignment =
wdAlignParagraphCenter
        ActiveDocument.Tables(1).Rows.Add
        ActiveDocument.Tables(1).Cell(ActiveDocument.Tables(1).Rows.Count,
1).Range.InsertBefore "Total words in Document"
        ActiveDocument.Tables(1).Cell(ActiveDocument.Tables(1).Rows.Count,
2).Range.InsertBefore Totalwords
        ActiveDocument.Tables(1).Rows.Add
        ActiveDocument.Tables(1).Cell(ActiveDocument.Tables(1).Rows.Count,
1).Range.InsertBefore "Number of different words in Document"
        ActiveDocument.Tables(1).Cell(ActiveDocument.Tables(1).Rows.Count,
2).Range.InsertBefore Trim(Str(WordNum))
        System.Cursor = wdCursorNormal
     '   j = MsgBox("There were " & Trim(Str(WordNum)) & " different words
", vbOKOnly, "Finished")
    Selection.HomeKey wdStory

End Sub