Question : prevent user from accessing specific form

Hi,
I need to prevent some network users accessing specific form .
i have private sub form_current() code that can show me the network username ( the value of getusername inserted into txtUserName control .
i need a way to tell the form on form_open or load or watever that if the value of the txtusername is "Mike" for example (the name taken from tblUserName) then let him in.
if the value if txtusername if null .then msgbox "u don`t have permission to see this form")
   

Answer : prevent user from accessing specific form

I'd say it's related to the
Cache-Control: max-age=-5843492 line

just a guest: can the expire value be the date of your last restart ?