DNS problems on LAN between iMac and XP

I have a customer site where a server app runs on a PC running XP Pro and the client app runs on several othe PCs running XP Pro. There are also a couple of iMac computers on the same LAN which run the client app. As far as the server/client app goes, all works fine. We have setup a share on one of the iMacs. A scheduled batch file copies the 2 data files from the server PC to the iMac share after hours. This has been working fine. The iMac user checks the date/time of the data files in his share each day to make sure the overnight backup (copy) worked.
If for some reason there was a problem, he can look across the LAN from his iMac and pull a copy out of the server into his folder. This has also been working fine for some time.
Recently, the overnight copy stopped working. As far as we know, nothing has been changed by anyone on either of the computers in question - except for possible auto s/w updates.
Now when the user tires to open the server from his iMac, it fails to connect. The server PC appears in his list of shared computers. The same thing happens if he tries to connect to any of the windows PCs. If, we try to browse to the server name in his browser, we still cannot connect, but if we put in the IP address of the server, we can connect straight away.
The IP address of the server has not changed.
It sounds like a DNS problem within the LAN between the iMac and the PC??? The ADSL modem/router provides DHCP services and it is the default gateway for the iMac, which also can connect happily to the net. The iMac can also connect to the server from the client app, as it always has been able to.
How can I refresh the DNS within the LAN? Or is there something else that I might need to do here?
Background info: The site is a medical practice. The server/client app is the practice managemnet s/w and was originally Mac only s/w. The site was having probs finding local support for Macs. The s/w was re-written for Windows. At that point I was asked to provide a bunch of PCs and setup the new LAN with the new s/w (about 5 years ago).
One of the Doctors later bought a new iMac when his replacement PC eventually died, because he never really liked the PC. I hav very little recent knowledge with Macs, but he user is fairly competent.
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first off, power cycle your modem / router
Then flash the cahced DNS on the imac. From the terminal window, run this command:

dscacheutil -flushcache

After doing that, restart your modem again, followed by the computers in question. Then see how you go
Where is your internal DNS server?  Do you have one?  This sounds like  classic NetBios name resolution issue.

Start by going to the Mac and running nslookup from a terminal window.  See if any of the PC names can be resolved to IP.  If they can't, that is is your issue.

I would suggest setting up an internal DNS server.  Do you have a Windows Server there?  Or an OSX server?  Or a Debian server?  Any server?

gregmiller4itAuthor Commented:
Naycam, I'll give it a go today & let you know what happens.

Wdurrett, the 'server' at this site is just a PC running Windows XP Pro. It runs the 'server' side of the Practice Management software and all the rest of the computers run the 'client' side. It is basically a Windows workgroup (no domain) with a couple of iMacs thrown into the mix (just to make it more complicated for me!). The only other server-like device would be the ADSL modem/router. It acts as a DHCP server and I've always assumed that it takes care of DNS as well as part of the dynamic IP settings. It gives the computers its own internal IP address ( as the DNS server. Then forwards DNS requests to the DNS servers it has received from the ISP.
As far as internal DNS, I've never had to worry about that before because it usually just seems to happen.
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In a work group setting, you need to make sure one machine is the Master Browser.  It will hold all the computer names for the work group.

For the machine that is always on (the "server"), make sure the NetBios over TCP is enabled and change the following registry key to TRUE.

Key: SYSTEM\CurrentControlSet\Services\Browser\Parameters
Type: REG_SZ
Name: IsDomainMaster
Value: True

If it does not exist, create it.

This should solve your issues of browsing by name.
Also check if the DSL router has any DNS settings that could have been reset/changed.
Is the Mac's set to be in the same workgroup?

System Preferences -> Network -> Buildin Ethernet* -> Advanced: WINS

* or AirPort if you use that
gregmiller4itAuthor Commented:
Yes. The Mac is in the same workgroup. When I 'view workgroup computers" from any of the PCs, you can see/browse/open all of the PCs on the network and also the 2x Macs.
When we browse from the Mac, we can also see all of the PCs and the 2x Macs on the network. We can open the other Mac from the Mac in question, by clicking on the machine name in the list. When we click on any of the PCs in the same list (as seen from the Mac) we get a connection error. If we try to browse to a PC share from the Mac (eg. //server/sharename) we get a connection error, but if we browse by IP address it works (eg. //
So, to recap, we can see the PCs by name but can't open them. We don't see the PCs in the list by IP address until we specify the IP address, then it stays in the list and we can open it by clicking on it.

We're still stumped as to why this is and how to fix it.
gregmiller4itAuthor Commented:
Things have not changed much since my last post. We still can only access Windows PC from the MAc by IP address but not by NetBIOS name. We have implemented a work-around by working from the PC end instead of the Mac end.
I am going to close this and award some points for assistance.
Thanks guys,

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