Look here guys, I have some good news as well as some bad news.......
I can connect to the XP PCs after I renamed them and the Win7 lappy.Now they are connected to the network WORKGROUP.
Here is my previous network diagram:

Problem is I have to connect the Win7 lappy to the switch thru a LAN cord(RJ-45). And when I go back to WiFi-n the problem is back again.
Here is the new Network diagram:

Now the question is - Why can't I access the network thru the WiFi-n:
The lappy is a Samsung N148 Plus running Win7 Ultimate and a Broadcom 802.11n Network Adapter.