Networked Macs see Windows server, but suddenly not the internet

Question

My girlfriend works at a small business running about 10 PCs and a Windows server. Also on the network are about 15 VOIP telephones. She's had an emac on the network working fine for a few months. It was connecting to the server and the to the internet. Then one day, no more internet. (None of the PCs are experiencing this problem.) Coincidentally, the next day a new Mini arrived. We set it up (to 10.4.3) and it saw the server, it saw the internet. Everything was beautiful and then poof. No more internet. It was working when she left work, it even received POP mail between her leaving last night and coming in this morning.
(I don't think this is the culprit, but it's worth mentioning that first thing this morning, she installed Microsoft Office. After she installed Office, the internet no longer worked. She didn't check her internet connection before installing the new software, but I suspect this is not a related issue. The emac does not have Office installed at all.)
The macs have been restarted and put to sleep and then woken up. All of the PCs get their internet connection after routing through the VOIP phones and so do the macs. We've taken the phone out of the equation, but still no luck.
Sorry this is such a vague description of the problem, but I don't know so much about inter-platform networking, so I'm not sure what to look for. And I don't know anything about TCP/IP or other networking protocols, so if there are any clues in the packets, I wouldn't see them. The most puzzling thing here is that it was working fine, and after seemingly nothing has changed, now it's not.
  Mac OS X (10.4.3)   macs on a PC network

Answer 1

It looks like your getting an address okay. Have you tried out a variety of applications to see if it only happens on certain ones? (eg, web browser but not iTunes) If it's okay to run that at work, anyway
Some explanations of the gibberish below:
en0: That's the short name for your network card. If you have multiple cards or other manufacturers this can be different but it's fairly standard. There will also be another device called lo0 displayed usually. This is called a loopback device and points to you machine. It will have the address 127.0.0.1 It's a good way to test servers or other things running on your machine, by connecting to that address. It's not really useful in this case, but knowledge is power
The flags indicate what the status of the network card is. It's up, and running normally for most networks. The mtu setting indicates the size of packets of data transmitted by your network card. 1500 is fairly common, but some networks or ISPs may note a smaller size in their support pages. If no one has been monkeying with your network recently, assume this is okay.
inet6 is support for the eventual switch to a better internet addressing standard. nothing to worry about here.
inet 192.168.50.54 is you address on the local network. So you're getting an IP at least. If a Windows machine on the network has taken that address as well, that can cause problems. It shouldn't happen, but it does occasionally. The windows box will typically spit up a warning dialog on the screen pretty quickly if that happens. I haven't run into it on my mini yet so I'm not sure if Internet Connect will pop up the icon with the lightning symbol or nor.
The netmask is a hexidecimal number showing the range or "scope" that your subnet (the area of the network you're in) occupies. FFFFFF80 is equal to 255.255.255.128. In this case it means that your subnet is restricted to a range of 127 addresses. If your address and the address of your internet gateway weren't in that range together you'd have a problem, because they wouldn't recognize each other as being part of the same subnet.
But the broadcast address would indicate that they are. Both fit within the range of 192.168.50.1 and 192.168.50.127
If those machines couldn't see each other then instead of your 192.168.x.x address, you'd see the 169.x.x.x address I mentioned, and would be off the network.
So, now that you've been bombarded with all that, it looks like you're okay on the local network so now the task is finding out why you can't see out.
Try a few different apps and see if they succeed, like running the software update, etc. If they work, then it's possible your browsers may need to be configured to use a proxy server to get to the net. That's a lesson for later
Here's my ifconfig info:
en0:
flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAS
T> mtu 1500
inet6 fe80::20d:93ff:fe78:6496%en0 prefixlen
64 scopeid 0x4
inet 192.168.50.54 netmask 0xffffff80
broadcast 192.168.50.127
ether 00:0d:93:78:64:96
media: autoselect (100baseTX <full-duplex>) status:
active
supported media: none autoselect 10baseT/UTP
<half-duplex> 10baseT/UTP <full-duplex> 10baseT/UTP
<full-duplex,hw-loopback> 100baseTX <half-duplex>
100baseTX <full-duplex> 100baseTX
<full-duplex,hw-loopback>
I don't see any "169" , but I don't really have any
idea what else is going on here. Any help would be
really appreciated. It's so frustrating when you
finally get a Mac in a PC office and then it doesn't
work.