Credo che questo vi sia utile!

<%
dim os
os = request.servervariables("http_user_agent")
if instr(os, "Windows 95") > 0 then
response.write "Windows 95"
ElseIf instr(os, "Windows 98") > 0 then
response.write "Windows 98"
ElseIf instr(os, "Windows NT 4.0") > 0 then
response.write "Windows NT 4"
ElseIf instr(os, "Windows NT 5.0") > 0 then
response.write "Windows 2000"
ElseIf instr(os, "Windows NT 5.1") > 0 then
response.write "Windows XP"
ElseIf instr(os, "Windows NT 5.2") > 0 then
response.write "Windows 2003"
else
response.write "qualche altro OS al posto di MS Windows " & os
end if
%>