Windows 2000 generates excessive traffic when connected to a BT 5861 Router

I support a small chairty that has a network with a diverse range of desktops connected to an NT4 File/Print server and a Mercury Email Server. The desktops are: 95, 98, 2000 and Apple Mac X. We have recently changed our internet connection from ISDN to Broadband via a BT 5861 Router. All worked well under ISDN and all the desktops could access the internet. Our broadband connection is Pipex Managed Service via a BT 5861 Router which is configured to be the DHCP and DNS Server and is NAT compliant. The router is pluged directly into the Switch panel.

All machines can access the internet via this router and via this configuration. However, after a few minutes the Windows 2000 desktops start to generate an excessive amount of traffic on the LAN (about 100 packets a second) and stop all other desktops accessing the internet - but the LAN access appears to be unaffected. The Switch panel shows the lights flashing for the 2000 desktops and the router only - the others flash very slowly (as to be expected). If I take the router off the network then the 2000 ports continue to flash. If I reboot the 2000 desktops (with static IP addresses) and leave the router off the network then all is well. If  I put the router on the network but do not boot the 2000 machines then all is well and everyone else can access the net. Reboot the 2000 desktops and the flashing starts again about 5 minutes (don't have to log the machines on).

Why do the 2000 machines stop the internet connection? Is it the router that is at fault? Should we upgrade to XP (did a one-off test and added an XP desktop onto the network and this worked OK)? We have tried everything over the past 6 weeks without a solution.
cheeserightAsked:
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svenkarlsenCommented:
Could sound like virus: have you scanned for virus, have you upgraded your anti-virus software ?

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svenkarlsenCommented:
Also check for add-ware, - use e.g. Add-Aware:

http://www.lavasoft.de/software/adaware/
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cheeserightAuthor Commented:
Ironically, installing the anti virus software was the next item on the list. They have bought the Symantec Antivirus Corporate Edition (8.1) and I have never seen anything so complicated to install. I will try installing this either Thursday or Friday this week and will get back to you. I always assumed that viruses attacked all Windows versions and not generally that selective.

I will download the Add-Ware product and see if that throws up anything interesting.

In the meantime, many thanks for the suggestions.
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svenkarlsenCommented:
have fun ;-)
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cheeserightAuthor Commented:
It worked. Each of the 2K machines had the Welch virus. Removed them and the network traffic stopped immediately. In addition, there was a lot of adware bits and pieces. Removed them all.

Can't thank you enough. The girls in the office are thrilled to bits. You have more than earned your points.
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