Connecting to a PC server from an iMac

I've recently installed a new iMac at my office, which before only had PCs - all connected to a PC server. I am connected to the server through an ethernet cable and know that the connection is fine since I am able to connect to the internet and printer without any problems whatsoever.

The trouble arises when I want to access shared documents on the PC server (titled "alfserver", which you can see in the pictures below). Sometimes the icon shows up in Finder (see pictures 1 & 2), but sometimes it simply doesn't show up at all (picture 3). No reason. The cable is plugged in (internet & printer works fine), but I can't find the server anywhere. I've tried placing a shortcut to it on my desktop to see if that helps, but that too only shows up on my desktop when the server shows up in my Finder window - ie. it's of no help.

Anybody have any ideas as to what is happening and why I can't always connect to it? It's highly frustrating when I'm working and need to access documents and it simply doesn't show up anywhere!

My details in case anyone needs them:
3.1 GHz Intel Core i5
8GB RAM 1333 MHz DDR3

From the Go menu in the Finder use Connect to Server.

In the next window type:

smb://alfserver.ip.address

eg if alfserver's ip is 192.168.1.10 smb://192.168.1.10

Enter

The next window will show you a list of the shares on Alfserver

You can also get to connect to server with cmd+k

Even though it doesn't make the servers appear in Finder, this still manages to connect to alfserver somehow! Thanks so much