windows 7 - Internet speed very slow only in WiFi and in just one laptop

I have Xfinity internet at home. All the devices (a couple of laptops, phones, tablets, etc) are connected via WiFi, except a PS3 which is connected via ethernet cable.

A few days ago we got the Xfinity gateway (router+modem) replaced because we had problem with it. I don't know if it's a coincidence, but around the same time I started to notice very slow internet in my work laptop. I've been doing speed tests in all the devices and they all seem fine (normal speeds) with the exception of this laptop (very slow speed).

I've been doing some tests:

1. If I connect the laptop to the router via ethernet cable, the internet speed is normal again.


2. If I boot the same laptop with Windows 8 (I have it with dual boot Windows 7 and Windows 8, but I usually use Windows 7), and do a speed test, the speed is normal again.


3. I've restarted the router/laptop several times and speed via WiFi still very slow


4. I've removed the WiFi network from the remembered networks of Win7 and connected to it again as if it was new network, and speed via WiFi still very slow

By slow speed I mean around 2Mbps when normally I would get around 35Mbps. The upload speed doesn't seem to be affected by this problem, it's usually 6-10Mbps.

Any ideas how I can get my laptop to get the same speed as always?

EDIT

So following the advice given in the first answer I reinstalled the drivers of the wireless card. I've been doing some tests with it and the rest of devices, and it seems that now I'm getting more speed that before, but the download speed still slower than expected. Although it fluctuates quite a bit, most of the results I get are around 10 Mbps, when before they used to be around 35 Mbps. Besides, it seems that in the rest of devices now it happens also the same (it's hard to tell because of how much the results change on each test). Can it be a hardware problem of the router/modem? Is there any way to test that? I called Comcast the other day and everything seemed fine on their end.

Answer

Because you mention that it works better in Windows 8, it could be a driver problem. Make sure you have up-to-date windows updates, including optional updates which sometimes have vendor specific hardware updates. Try locating your network adapter in your device manager, right clicking the adapter and select "Update driver software..." If this is already up to date, try uninstalling and reinstalling the driver. If this is a dongle and not internal card try searching for the driver directly from the manufacturer.