Win Xp Strange Wireless Problem

Problem:

Have an 11 meg internet connection,(download) But I cant get over 5 meg
download (speakeasy test))when using my wireless connection. (using Linksys
11b/g/n router.) PC wireless adapter is at top bitrate of 54mbps.

Info:

Running windows XP sp3 all OS updates on a Dell Latitude d520. The wireless
card is an 11b/g Dell Wireless 1390 WLAN Mini-Card with the latest drivers.
When I go hard wired straight to my router I can get the full download
throughput of 11 meg. I also have tried a Ubiquiti 11b/g wireless PC Card
that connects at the top bitrate but can only get 5 meg download as well.
Here's the kicker: My daughter has a Dell Studio Laptop. She can connect
wirelessly at a bitrate of 54mbps to my router and she gets the full 11 meg
download on speakeasy test. So that really blows my mind. Its like my laptop
is forcing B only mode speeds regardless of what kind of wireless card I use
even though its connects at G speeds.

What I have done:

Tinkered around with some of the advanced properties of the wireless driver.
I have ran all the diagnostics that are included with the driver utility and
everything passed. Thats about it. This is really got me stumped.
I have ruled out a router problem and it cant be a tcp/ip problem with my PC
since I can get the full speed over a hard wire.

Anybody else run into this before?