Most Powerful computer, but slowest internet connection in the same house.

My computer is the most powerful in my home. It has the most RAM, GPU power, CPU power, ect. It's got all the fancy bells and whistles compared to the others. However, despite all that, my mother's laptop which is old, still runs on XP, filled with junk files, and runs much more slowly on the CPU power has a much stronger internet connection by up to 5x.

Here's some basic connection information:

• Wireless connection on both computers.
• Wireless Connection Speed Status on my computer reads 11/18 mbps (it changes between the two). It used to be a mere 2mbps, but I changed the channel in router settings to the default number (11) and that boosted it up.
• Speedtest.net has my download speed at 4-5mbps on average. Upload is 3-4 on average.
• Mother's crappy laptop gets nearly 20 mbps for download speed and 8-10 upload speed on Speedtest.net (54.0 mbps on Speed Status)

Here's some information on what I noticed and the steps I have already taken:

1. As stated above, often it would drop so slow that watching Youtube videos above 360p was impossible. I found I needed to change my router channel on my PC since there was interference. That helped my 0.5-2.5 mbps internet jump up to 4-5mbps (according to speedtest.net).
2. I have ran a virus scan/malware scan/defrag/computer cleanup/registry cleanup/ect.
3. Firewall is off and anti-virus is turned off.
4. I have attempted using Google's free DNS server.
5. I have also attempted a simple resetting of the router and restarting of my PC.
6. My Wireless Network Adapter drivers are updated to the latest version.
7. My Wireless Network Adapter is a Broadcom 802.11g (easily the oldest piece in my PC, but will only replace if it's discovered to be the true problem, which is why I'm asking here before I buy a new one).
8. Distance from Router is about 30ft. Mother's laptop sits right next to it, though suffered no significant internet speed drops when taken further away from the router, even when further from my PC's distance.

My computer is a Windows 7 64-bit
4GB RAM
GeForce 460 GT
Core 2 Quad Q9650 @ 3.00 Ghz