Excessive Data Consumption

Excessive Data Consumption

I need some help, would be thankful if someone give me the idea for this.
We've a wireless internet connection in our office and the day since then
our office has switched from a dynamic IP to a static IP, the data
consumption has increased more than doubled. 1. We have a 20 GB monthly
limit but do not have any internet consumption at our office except the
emails we receive and the anti-virus or software updates (which are
manual). The only thing we do is to connect the software on the machine
via remote desktop (as we have 2 office locations), which i don't think,
could be a cause of excessive usage. 2. We have even put a restriction
recently so that only the IPs (which we have added in our modem firmware)
could connect to the internet, but event then within a week we have got a
notice from the provider that we have exceeded our 50% data consumption
limit. 3. Someone has told me that the modem has been configured to use
extranet (don't know how to check that) which should be reset to use
intranet to avoid this extra usage. By this way, we won't be able to use
the static IP, but our usage would decrease gradually (not sure about
this, would that help?). We are using windows 7 on all PCs Any ideas?