Well, that's a start. The servers are good; Dell poweredge's are superb. I actually have a poweredge 2950 here at home running Server 2008RT SP2 Pro.
So, I take it that most if not all of your 400 XP PCs are custom-built using Intel Motherboards. That's good, at least you've got solid Motherboards. You will need to run those hardware diagnostics on a test sample as I indicated (3-10 PCs) in a standalone LAN configuration to really nail that down. Another thing I thought of, are all 400 of those XP machines at SP3 level? If not, that can wreak havoc having a mix of XPs running different Service Packs. Make sure your test setup all XP machines have SP3 on them!
I would definitely get rid of Kaspersky on the test machines;
REMOVE; the Kaspersky is russian, nothing wrong with that, but the product is never in the top 10 of anyone's antivirus or Internet Security suite on the e-zines annual review site. I would recommend you use
Norton, Avast, McAfee, Trend Micro, or Panda. I have tested out all of these for both home use and enterprise corporate use. Norton is still arguably the best, but the most expensive. Last enterprise license for IS I got in 2000, ran about $20k or so. If you like, for your testing purposes, remove the Kaspersky (it's terrible), and download Avast AV Free 2015. That will work for your test, and has a very thin client footprint--uses much fewer resources than Norton or the other guys, and it won't cost you anything. It also has a very intelligent firewall which may show that your VNC will work fine on XP with Avast. If it works with Avast, it will work with any of the other top guys I mention including their Enterprise corporate network licenses. I have business clients who use logmein, which is a more modern and cheaper version of VNC with the Avast Internet Security and the Free version, and they work great--especially with my PCs which are on wifi connections.
You're network is about the size of my last network I managed-680 computers--as I mentioned. However, I've worked on networks as large as 70,000 computers (FedEx-Kinkos), so I've got lots of experience doing this.
It's also very good that your Motherboards are running Intel dual processors rather than the AMD or VIA dual processors, as Motheboards using those processors had many documented problems on applications such as remote-in apps (VNC, Logmine, GoToMyPC, etc.), and especially ERP platforms in the enterprise such as SAP, CS/3, Oracle, and others using XP-era machines. Vista & Win7 did a much better job at using these "gaming" type processor-based Motherboards than XP or Win2k for instance.
Do any of your network segments contain these Intel-based XP clone PCs running alongside the Win7 machines? On the same subnet? If so, this would be another job for the sniffer to check your packet traffic on that subnet, it could give you a clue as to why the XP networking is failing and the Win7 is not. However, this does not preclude that you still have to run complete Hardware diagnostics on each of those XP clone machines in your test LAN setup.
THIS IS CRUCIAL!!! Also, as part of updating those old XP clone boxes, you'll need to visit the Intel website and check each and every PC in that setup LAN has the latest version of BIOS and Chipset drivers listed on the Intel support site. I would do this before I started sniffing packets for irregularities on the test or production subnets. If you don't have access to a sniffer, you can buy the software separately and run on a dedicated laptop or maybe your work laptop. The sniffer machines are expensive and can be upwards of $15k, and to get an expert (IBM Level 4 or SME or better) who knows how to use it can cost you quite a bit more than the cost of the machine. However, in the US, we have many consulting firms who rent the equipment along with the expert sniffer engineer on a hourly/weekly/monthly/yearly basis. Don't know if you have any people like that in your Country, but you could always contact IBM and they could fly someone in for you.
(won't be cheap!!). Just discussing some ideas and brainstorming with you here, but it's important to know that there were significant changes made to the Windows networking and system files (both API and DLL library levels) in Win7. Win7 really does a much better job at networking in the enterprise in a domain-based server LAN environment than XP, Vista, or even Win2k ever did. The real benefit of course really comes if your clients are using Win7 Pro or Win7 Business or Win7 Enterprise workstation clients, and they are really designed with all the extra management features such as SNMP and Remote Desktop image stores etc. Most medium and larger sized companies have gone to Win7 probably just as you started to do, but ran out of money.
In today's economy, upgrading that many computers represents a real financial challenge. Where I live, in a rural Southern California resort town; businesses are remiss to attempt a complete overhaul of their network and the OS workstation clients at the same time. Our local hospital has been upgrading since I moved up here 4 years ago from XP to Win7. Some of their offices and doctors are still running XP machines, some on Win7. They don't have the budget to do it all at once. I wouldn't be surprised to see them to full migrate to Win8.1 even by 2025. They are typically 10 years behind the large urban cities like LA or San Diego or San Francisco. I'm sure this is part of the challenge you are having.
Antother thing you can do, is to consider changing from VNC software client to something more modern and a more thin-client based solution (more modern) such as Logmein or GoToMyPC. There are others as well, but these have worked well for my Clients in medium to large size companies; 500-5,000 computers and up. The enterprise licensing is cheaper, and the thin-client design takes up less resources on your aging fleet of 400 XP machines; XP can still run either of those thin-clients without breaking a sweat. Especially given your XP Motherboards are running dual-core CPUs not single-core (nightmare!).
Hope that provides some more insight. Gives you several more things to try out. The most important thing you can do really, is setup that test LAN I mention. That is the single best tool you can use to justify further upgrades to older PCs, and older software apps. Bear in mind that according to Microsoft, of the 800 Million computers being used currently worldwide on the Internet, 36% of them are still using XP--so you are not alone with your upgrade issues.
Keep us posted on your progress.
P.S. Since you don't have a network topology diagram or map, it sure might be helpful for you to get one or create one of your own. That solves a lot more problems than you would ever think. Take it from someone who knows!
Best,
<<<BBJ>>>