Can't Access File Server From Laptop

I've got this problem about accessing a file server in my work office.

First, I used to have a desktop PC connected via ethernet cable to a wireless router (there's only one connection in this office but there are two of us, so we set up a router), and I was able to acces the file server by typing "\SERVERNAME" in the RUN command windows. I brought my laptop and I connected it to the router via wireless. After I tried accessing the server the same way I used to (typing "\SERVERNAME") but I got the "Windows cannot acces \servername". Well, I said, I'll try by connecting via ethernet cable, but I couldn't connect either. The only way I can access is by disconnecting the internet cable off the router and plugging it in my laptop's ethernet port. But the issue is that we've only got one connection and some other people connect wirelessly too. I can't understand why if connecting an ethernet cable to the router's LAN port, I was able to access the server from my desktop computer but when I tried the same way from my laptop I could not.Internet connection works fine in my laptop. Networks settings are ok i think. My laptop has Windows 7 and my desktop has Win XP.