I've seen similar issues to this on this site and others but I believe that this is a unique (and very strange) situation and nothing I've seen so far has solved the problem. Apologies for the length, but I'm going to try to be as detailed as possible. My networking knowledge isn't particularly good, but I have a grasp on the basics (I'm a software engineer, so it helps to have some sort of idea).
I've been using a Windows XP deskop (HP dc7800) for a number of years. I've recently been given a newer desktop (Dell XPS 8300), which I've installed Windows 7 Pro onto. For the past couple of weeks, I've noticed the internet connection speed on my Windows XP desktop gradually decreasing. I was told that it's probably a hardware issue and seeing how I was getting a new desktop, I didn't let it bother me. I've just got Windows 7 installed and I'm having exactly the same problems on there as well.
I don't have any problems connecting to other systems on the LAN. I can access network drives just fine, and with no speed issues. My Windows XP desktop is at the stage where it can barely connect to the internet - it takes around 2 minutes just to load Google.com via Firefox. My new Windows 7 desktop is a little faster, but it's still very sluggish. The main problem is when I try to download anything, via either of the desktops (for example, I've tried to download Firefox and Chrome on my Windows 7 desktop). The download speed is hovering between 1 and 5 KB/s. Consequently, even trying to run Windows Updates is impossible.
Both PCs are connected to the work LAN via ports in my office. There are another two people in the office and their PCs (they have a desktop and a laptop each) are fine. literally the only two PCs with a problem are sitting on my desk.
Things that I've ruled out
External network connection
Obviously this isn't an issue as the other people in the office aren't having a problem. If there was a general problem with the connection, we'd all have the same problem.
The cables from the ports in my office to the server room are fine. If I plug my own personal laptop (XP) into either of the ports, my internet connection speed is as fast as I'd expect it to be. Likewise, if I plug my new desktop straight into the switch in the server room, it's still ridiculously slow.
My domain account
My XP desktop logs on to the domain in the office. My Windows 7 desktop doesn't - it uses a local account, and it hasn't been connected to the domain yet. I've also tried other domain accounts on my Windows XP desktop and they all get the same problem. Likewise, I've signed on using my domain account on one of my colleagues' (working) desktops and I have no problems with connection speeds on their desktops. Given the combinations of accounts and desktops, that rules out an account issue (i.e. no throttling set up against my account).
All PCs in the office use the same internet/network settings. We're all set to obtain IP and DNS addresses automatically, and running ipconfig /all on both my desktops, my personal laptop and one of my colleagues' desktops shows that they're all practically the same (aside from IP address, MAC address etc. that will obviously differ). The rest of the settings (gateway, subnet mask etc.) are the same.
This is turned off on my Windows 7 desktop and not even an option on the Windows XP desktop.
My Windows 7 desktop is a clean install. I've literally installed Windows 7, created a user account and logged on. That's all. I've also recently run a full scan on my Windows XP desktop and that came up clean.
I've flushed my DNS cache, disabled and re-enabled the connections etc. and run a whole series of other random network related command line commands that I've found on various forums.
This isn't a hardware issue. I've used an Ubuntu LiveCD on my Windows 7 desktop to boot into Ubuntu (without installing) and connected to the internet just fine. Download speeds are as expected (i.e. working), which rules out it being a problem with the network card on that desktop.
I've made sure that both desktops have the latest network card drivers.
What does that leave? All I can think of is that there's an issue somewhere in the server room configuration itself - for example, I'm wondering if someone has changed some routing information on one of the servers without logging it anywhere, or something along those lines. As I said, my networking knowledge isn't great, but I've pretty much run out ideas and I can't find anything else online that helps, having tried most suggestions. I'm based in a small satellite office, and I've been going backwards and forwards with our support team (in our main office) to try to resolve this, but they're not on site so it's a case of try to fix things myself if possible.
Any suggestions would be gratefully received. Any time I come up with an idea, a bit of testing usually proves that it isn't the case and I'm still no closer to having a working internet connection!