Dell Wireless 1520 PCIe card - Very slow connection

Hi

First post, please forgive me if posting protocols breached.

I have a Dell Precision T3500 tower running Windows 7 Ultimate. This was built and shipped in March 2011 equipped with a EMEA Dell Wireless 1520 PCIe card, which has a wireless antenna attached to the port. The PC is about 30ft from the wireless router, a BT Home hub.

From outset I noticed that I was gettimng very slow connection speeds to the internet, but supposed this was because our broadband was using a standard BT line with copper cables, and we are some 3 miles from the BT exchange. I was getting speeds of around 300-500kb/s. Rubbish, I know.

BT have just finished installing BT Infinity in my area, offering 'up to 40mb/s' speeds. I upgraded on Friday of last week.

Here's the problem: we have three laptops in the house and they are now all getting between 15mb/s-20mb/s speeds, including my Dell Inspiron laptop which is using a Dell Wireless 1501 Mini Card to connect wirelessly. Sat next to this laptop is my Precision tower, with the wireless antenna. However, the PC is getting a speed of only 3mb/s.

The modem and router are obviously working fine. Forget 40mb/s, I am quite happy with 15mb/s. But why is the PC wireless speed so poor?. For a brief five minutes or so on Friday the PC DID get 15mb/s but not since then. I cannot see any way to configure the wireless card to increase the speed.

I tried disabling the Dell network adaptor, and installed and plugged in a Belkin USB wireless adaptor (on which my wife's PC had achieved 15mb/s) but this slowed the wireless even more - to around 100kb/s.

Am I obviously doing something wrong? Could the Dell wireless adaptor be faulty as it has never achieved decent speeds)? Any other suggestions please?

Thanks