Windows 7 machine makes router drop ALL wireless connections

Some background:

My home network consists of my desktop computer, a two-month old Windows 7 (x64) machine which is online most frequently (N-spec), as well as three other Windows XP laptops (all G) that only connect every now and then (one for work, one for Netflix, and the other for infrequent regular laptop uses). I used to have a Belkin F5D8236-4 wireless router, and everything worked great.

A week ago, however, I found out that the Belkin absolutely in no way would establish a VPN connection, something that has become important for work. So I bought a Netgear WNR3500v2/U/L. The wireless was acting a little sketchy at first for just the Windows 7 machine, but I thought it had something to do with 802.11n, and I was in a hurry so I just fished up an Ethernet cable and disabled the computer's wireless.

It has now become apparent, though, that whenever the Windows 7 machine is connected to the router, all wireless connections become unstable. I was using my work laptop for a solid six hours today with no trouble, having multiple SSH connections open over VPN and streaming Internet radio in the background. Then, within two minutes of turning on this Windows 7 box, I had lost all connectivity over the wireless. And I was two feet away from the router. The same sort of thing happens on all of the other laptops — Netflix can be playing stuff all weekend, but if I come up here and do things on this (Windows 7) computer, the streaming will be dead within ten minutes.

So here are my basic observations:

• If the Windows 7 machine is off, then all connections will have a signal strength of very good or excellent and a speed of 48-54 Mbit/s for an indefinite amount of time.
• Shortly after the Windows 7 machine is turned on, all wireless connections will experience a consistent decline in speed down to 1.0 Mbit/s, eventually losing their connection entirely.
• These machines will continue to maintain > 70% signal strength, as observed by themselves and router.
• Once dropped, a wireless connection will have difficulty reconnecting. And, if a connection manages to become established, it will quickly drop off again.
• The Windows 7 machine itself will continue to function just fine if it's using a wired connection, although it will experience these same issues over the wireless.
• All of the drivers and firmwares are up to date, and this happened both with the stock Netgear firmware as well as the (current) DD-WRT.

What I've tried:

• Making sure each computer is being assigned a distinct IP address (they are).
• Disabling UPnP and stateful packet inspection on the router.
• Disabling Network Sharing, SSDP Discovery, TCP/IP NetBIOS Helper and Computer Browser services on the Windows 7 machine.
• Disabling QoS Packet Scheduler, IPv6, and Link Layer Topology Discovery options on my Ethernet controller (leaving only Client for Microsoft Networks, File and Printer Sharing, and IPv4 enabled).

What I think:

It seems awfully similar to the problems discussed in detail in Weird one – Router Death by Vista (which was both the most relevant and concrete information I could dig up on the Internet). I still think that something the Windows 7 IP stack (or just operating system itself) is doing is giving the router fits. However, I could be wrong, because I have two key differences. One is that most instances of this problem are reported as the entire router dying or restarting, and mine still works just fine over the wired connection. The other is that it's a new router, tested with both the factory firmware and the (I assume) well-maintained DD-WRT project. Even if Windows 7 is still secretly sending IPv6 packets or the TCP Window Scaling implementation that I hear Windows Vista caused some trouble with (even though I've tried my best to disable anything fancy), this router should support those functions.

I don't want to get a new or a replacement router unless someone can convince me that this is a defective unit. But the problem seems too specific and predictable by my instincts to be a hardware hiccup. And I don't want to deal with the inevitable problems that always seem to take half a day to resolve when getting a new router, since I'm frantically working (including tomorrow) to complete a project by next week's deadline. Plus, I think in the worst case scenario, I could keep this router connected directly to the modem, disable its wireless entirely, and connect the old Belkin to it directly. That should allow me to still use VPN (although I'll have to plug my work laptop directly into that router), and then maintain wireless connections for all of the other computers. But that feels so wrong to me. What could the cause and possible solution be?

Clarifications:

• The Windows 7 machine is directly connected via an Ethernet cable to the router for everything above. But while it is online, all other computers' wireless connections become unusable.
• It is not an issue of signal strength or interference — no other devices within scanning range are using Channel 1, and the problem will affect computers that are literally feet away from the router with 95% signal strength.

Update:

After running a week or two with the two-router setup (the router in question used for wired connections and VPNs, with the other router plugged into it and set to broadcast), I tried the suggestion below of connecting a different Windows 7 machine and seeing what happened. I know a few router settings were changed during this time — the wireless broadcast mode was disabled, and I re-enabled UPnP. I also may have disabled or enabled a service or two on the problem Windows 7 machine while I was trying to setup OpenSSH. But when I restored the router to its wireless-broadcast configuration and plugged a freshly-installed Windows 7 computer into it, the NetFlix laptop didn't drop the connection.

Strangely, though, when I plugged the problem computer in as well, the connections were still stable. I don't know what has significantly changed about the situation — I haven't done a firmware or Windows update, but it seems unlikely that having the broadcast mode disabled for a while would cause it to work better, especially since I was having this issue right out of the box. Still, I haven't run any more extensive tests than trying to tax my network connection while a movie played through to completion in the other room.

I don't yet trust that the problem has just vanished, and I know I wasn't just imagining the symptoms, as they were very specific and consistent across a significant period of time. So I'll do what I can to recreate the original situation, or determine with more certainty that it really has abated, although this may take a while since everyone else is on vacation. I'll update this section whenever I can figure anything new out.

Update 2:

Well, it's been far more than a few weeks and the problem has never resurfaced. (It's amazing how easy it is to forget about something when it's not causing trouble.) This is very unsatisfying to me, as it means that there is no concrete solution, and I know other people have had this problem or something similar to it. I'm going to give the answer to the guy who suggested connecting a second Windows 7 machine just to close the issue — not only was it a good idea that I hadn't thought of, but the problems mysteriously vanished around that time. Maybe there's a connection, maybe there isn't. Thanks for all your help, everyone.