Networi Problem

I have four computers on a network - two wired connections and two wireless.
Three are working perfectly and can access all four computers.
One - a Medion with a wireless connection - cannot 'see' any of the other
three computers - although it can be 'seen' and accessed from each of the
others
The Medion can access the internet via the router
My Network Places simply opens with an icon of the router.
Any ideas, please - I have tried and tried!
D