how to automatically detect whether to use proxy or not

Question

here is the situation, I work with a bunch of non-tech sales people, they sometimes require to access a set of private services that should only work within the office.

so I set them up a ssh access using putty and explained how to use it in order to access local services... it was the hardest thing I've ever done. to try to explain a sales man about computer that requires more than two steps (DON'T DO IT, IT'LL KILL YOU)...

anyways, I'm just wondering if there is any possibility what so ever that:

1. whenever they are in office, their browser automatically uses direct connections for ALL websites.
2. whenever they are outside the office, their browser uses a proxy **ONLY** for our INTERAL services.

please note, the clients ( salesMan laptop) are only Windows