You have two options to dynamically "include content".
1) use the execute method wich will run the ASP page in much the same way as the include method.....
if x = y then
execute "myfile.asp"
else
execute "myfile2.asp"
end if

or 2) the less pleasant way is to use FSO to read the contents of the file then write it out to the client. this is no good if the file contains serverside code.