I believe that there are a number of issues that, when combined, can tip your system over the edge and convert you from a smiling satisfied customer to one who is tearing their hair out and kicking the cat....
At the moment my system is stable at 50Mbps and hopefully it will remain that way.
Below are some of the thought processes I went through, some areas of assistance I used and some of the issues that I was faced with. Also my own opinions and observations which you are free to ignore. I hope that this helps and does not confuse.
Firewalls
Firstly there is the PC resident Firewall. It seems that the Kaspersky firewall can cause problems. Remember that any firewall is intercepting the data traffic and checking as well as checking at the TCP/UDP levels. Depending on the machine processor power this has the potential to slow traffic and hence the resulting speed. If you consider that PCGuard uses the Kaspersky Anti-Virus engine as its base and works well where as Kaspersky struggles. Windows Firewall, which is incredibly simple and has no visible impact on the traffic.
It may be, and this is pure speculation, that the older versions of firewall software in Internet security packages may creak as they are continually processes patched code.
Also consider a clean install of the firewall.
Secondly there is the Firewall in the Router. The Draytek Vigor2910, which is an excellent Router, has a published throughput of 15Mbps and in normal operation achieves speeds higher than that. Draytek are unusual in publishing this type of data unlike many other Router manufacturers. Without that knowledge I would not have been aware of issues with the Netgear configuration.
Remember that it is possible for your PC firewall to conflict with your router Firewall.
The Router
The Router is important as well as its firmware level and configuration. The Netgear was abysmal until I managed to get its firmware upgraded and fully configured. The issues I experienced in upgrading the firmware meant that I could not configure the device properly.
Virgin should consider the standard loading of a configuration file on to their routers. Sky manages to do this with the Netgear routers that they supply. It would also allow the Router to be supplied preconfigured with TKIP security (although this should not be based upon the MAC address as that is identifiable in QoS packets).
It would be nice to have a list of routers that are known to perform well and their configuration parameters.
It would also be nice to have a router with Gb LAN capability. I now have a number of devices running through a 100Mb LAN which is going to throttle my 50Mbps even if those devices are not connected to the Internet and are only Intranet oriented.
All testing should be done with no other devices attached to the Router.
The Network Card
It may be that my Marvell NIC was teetering on the edge or that it is just and old and potentially damaged card (it is almost 5 years old). It was set to 100Mbps Full Duplex where as the Nvida nForce that is working well is set for full auto negotiation (as it is working I am not going to touch that parameter).
Both my cards are capable of 1Gbps but that seems irrelevant but must be remembered that a 100Mbps card is the minimum.
Cabling
Quality cabling is required. My first Telewest cable was a flat thing which still resides somewhere behind my PC. Avoid old cabling even if it is easy to find in the spagetti behind the machine. For control I would recommend a quality Cat 5e shielded or Cat 6e shield cable from a reputable supplier. Cat 6e is not expensive now (very cheap on Amazon, expensive at PC World).
If you have problems try to get a brand new cable or get the Virgin engineer to use theirs as at least that takes your equipment out of the equation.
Device Power Supply
A clean protected power level may have an influence. Do not expect consistency from a cheap power board daisy chained through other power boards. We all do it but when there are problems ensure that you have a clean power supply for all devices in the test cycle.
The Modem
It seems there can be problems with the modem, possibly in my case the line power level, which can push the configuration beyond its capability. This also applies to the upstream devices etc which are totally under Virgin's control.
The Browser Configuration
It seems that Firefox can cause some delays and I initially did all my testing on IE7. However, both can throttle depending upon their configuration and also interaction with your configured firewall. It would be nice if a non browser based test package was developed and made available.
The Test Sites
I used three test sites (two Virgin Sites and SpeedTest.net). You must be sure of the capability of the site you are downloading from as these can give a false picture without you are being aware.
A Control Environment/Configuration
I am surprised that the Virgin installation does not first test with controlled known equipment which is proven. This would be a simple pack of known router and known laptop.
Patience & Control
Network issues can take a long time to resolve even if you have the right testing equipment. Be logical and methodical only changing one parameter at a time and then evaluating that parameter against different changes. Document each step so that you know what you have changed.
Do not lose your temper. Not only does that make it difficult to understand what you have done to the system but also it makes it easier for the various help desk people to help you.
The Help Desks
You will probably call multiple help desks in order to resolve each of whom will believe that it is not their equipment at fault. Be firm, communicative but above all pleasant. Do not be offended if they tell you to do something that, you think, makes you look stupid. Remember they do not know you and have to deal with all types of people. If they ask you to do something which you think is irrelevant ask them, pleasantly, as to why that is required explaining why you do not believe that is an issue or a step that may assist resolution. Remember that, in the main, they know their stuff even if they are based in India or Central America. If dealing with those counties remember that English may be their second language and even if their first language they may have different ways of saying things and also thinking.
I would say that in this problem the Help desks of Kaspersky, Draytek, Netgear, Virgin Technical and Virgin 50Mb were great. Sometimes we went down dead ends but they put a lot of effort into it and were very pleasant. Thank you.
Google Help
Be circumspect of any "Google Search" based research. Yes it can help but it can also send you down really convoluted paths with changes which can have significant impact. If you do change key areas make sure that you can revert to the previous configuration and if you cannot revert then make a backup image of your machine before you make the change.
Time
This is not a fast approach. It took me 10-12 hours of solid work to obtain a stable performant system and I am glad, as I am sure that they are, that my family was away yesterday. Now they can come home and see me smiling.
Luck or Intuition
Heaven knows but it always helps. So if you are having problems then good luck.