So you're router is effectively at your ISP?
If you run ipconfig /all on the w7 machine & another how do they compare?
Really curious as to what could do this.
for curiosity's sake, try setting the w7 machine (statically) to the laptop's settings (disconnect laptop) and see what happens. If it changes, maybe manually set the gateway...are they on the same ip range?
Well as I just posted, the problem seems to be browser related after all somehow, since FF seems to be unaffected while IE & Safari are affected, what Safari and IE share that FF doesn't?
How exactly are you measuring the speed? Are you using something outside the application like NetMeter? In-application indicators (especially browsers) can be notoriously inaccurate.
Are you also using multiple connections to saturate your bandwidth, either by increasing your browser connections, or preferably a download manager so you can have multiple files from multiple sources downloading at once?
So what was it detecting automatically vs now??