Extremely slow internet speed after reinstall

I have recently had a hard drive failure so I was forced to reinstall my OS (Windows 7). After a fresh install, I noticed the internet speed became extremely slow (~15 minutes for downloading a 20MB file). I have installed all available windows updates and drivers.

I have a cable connection that is supposed to be 40Mbit, and this was the performance I was getting before the reinstall, as well. Between the modem and PC there is a SMC SMCWBR14S-N2 router.

I have other machines on the network, a PC with Windows 8 PRO which is connected by a lan cable and another Win7 machine that is using wireless lan. Both of these other machines show full performance (40Mbit) when downloading and also on speed tests.

What I have already tried:

• I assumed this might be caused by a faulty driver, so I tried three different NIC driver versions (Original driver from the Motherboard manufacturer website (v15.37) as well as the latest stable and beta driver from the Nvidia homepage (v15.57 and v15.58). None of these had any influence on the performance
• Changing the NIC to a different one. I had a PCI NIC lying around, so I installed it. Unfortunately still slow.
• Changed the cables.
• I also went to my ISP and traded my modem against a new one. No go.
• Swapped out all hard drives and reinstalled the OS
• Connected directly to the modem without a router

I also have Ubuntu installed as a second boot option, and experience full speed with it, with both NICs. If I go back to Windows, it becomes slow again. Here is a comparison from speedtest.net:

I have no special software installed. This is a fresh reinstall of windows and it worked fine before the reinstall.

Here are relevant system specs:

  Operating System                        Microsoft Windows 7 Ultimate 6.1.7601.18044 (Win7 RTM)
  CPU Type                                DualCore Intel Core 2 Duo E8400, 3000 MHz (9 x 333)
  Motherboard Name                        Asus P5N-D  (2 PCI, 2 PCI-E x1, 2 PCI-E x16, 4 DDR2 DIMM, Audio, Gigabit LAN, IEEE-1394)
  Motherboard Chipset                     nVIDIA nForce 750i SLI
  Network Adapter                         Realtek RTL8139/810x Family Fast Ethernet NIC  (192.168.2.100)

I am stumped as to what could be the case at this point. Please help.

To locate the problem, I would tried by order:

• to change RJ45 cable and switch cable to another port of the modem, and to try with ubuntu live cd => to remove external hardware issues.
• If you got your windows 7 image via utorrent, I would tried another one. => to remove potential trojan, spyware, ... issues
• I will use a software to monitor outgoing connection (windows tools like Little Snitch for Mac, most firewall can do it) => to remove potential trojan, spyware, ... issues
• Try to sature my bandwidth with ping command line and mesure it with bandwidth monitoring tool => to locate if the problem is only with TCP protocol or not
• Your Wireshark log shows TCP DUP ACK, I would tried to reduce MTU packet size (tools should exist to do it). => in case of weird communication between your network adapter driver and your cable operator modem. (It shouldn't be that because in this case it would have been worst.)
• Check my disk with a scan disk => in case windows had been installed on bad sector (?)
• (Check my RAM, with Memtest86 => shouldn't be useful here)
• With no other devices connected on the same network and behind a NAT, reinstall a default configuration with every update => If it didn't change, I would suspect a driver issue.

I've noticed that you have an ASUS MB. Are you running the AI Suite? More specifically are you running the Network iControl within that suite? In doing some research about this utility, I found dozens of complaints about internet being slow due to having the Network iControl turned on. The one symptom that you don't seem to have is upload bottleneck which was one of the symptoms that the Network iControl had, so this may not be relevant to you.

While this does not make sense considering that the Network iControl is supposed to help with prioritizing task associated with internet and network traffic, nonetheless there is evidence. I tried it myself because I was curious and I noticed the same issues others had experienced, but again, the issue usually affects both download and upload speeds.