OK, since there were no immediate suggestions, I wrote it myself.
For anyone else who wants it, find the following code around line
250 in default.asp:
if Cat_ID <> "" then
Cat_Name = allCategoryData(2,0)
Response.Write " <script language=""javascript"" type=""text/javascript"">" & vbNewLine & _
" document.title='" & chkString(Cat_Name,"pagetitle") & " - " & chkString(strForumTitle,"pagetitle") & "';" & vbNewLine & _
" </script>" & vbNewLine
After this code, insert the following:
' ###### BEGIN Greg's Add Forum Tree to default.asp?CAT_ID=xx
Response.Write " <table border=""0"" width=""100%"" align=""center"">" & vbNewline & _
" <tr>" & vbNewline & _
" <td width=""33%"" align=""left"" nowrap><font face=""" & strDefaultFontFace & """ size=""" & strDefaultFontSize & """>" & vbNewLine & _
" " & getCurrentIcon(strIconFolderOpen,"","align=""absmiddle""") & " <a href=""default.asp"">All Forums</a><br />" & vbNewline
Response.Write " " & getCurrentIcon(strIconBar,"","align=""absmiddle""") & getCurrentIcon(strIconFolderOpen,"","align=""absmiddle""") & " <a href=""default.asp?CAT_ID=" & Cat_ID & """>" & chkString(Cat_Name,"pagetitle") & "</a><br />" & vbNewLine
Response.Write "</font><BR></td>" & vbNewline & _
" </tr>" & vbNewline
Response.Write " </table>" & vbNewLine
' ###### END Greg's Add Forum Tree to default.asp?CAT_ID=xx