Best Way To Display Warning Box Network Wide For Old Browsers

I was wondering if anyone has a suggestion for bringing up a "warning box" network wide for those using browsers no longer supported.

For example when a, Windows IE 7 XP user pulls up a site, a notification box says something like,

"Your computer is using XX OS on Browser XX. This website was created using components that are supported by modern browsers, etc,etc. please update blah blah...

I have a client that wants something for old browsers. It's specifically related to the popovers but I just assume pop up a window to notify any old browser/OS users.