Windows 7 internet speed slow on wireless, but fast on wired

I have two Windows 7 Laptops that connect to a Motorola SURFboard eXtreme cable modem SB6121. Suddenlink (cable provider) recently upgraded our network which gives 50 MPS speeds or more. I can only get 50 MPS when I disable wireless on my laptop and connect with a direct cable. Otherwise I get 20 MPS maximum speed. I borrowed a Windows 8 PC this weekend and was getting 51 MPS speed using wireless through the same modem. Any ideas of what could be causing this issue?

Thanks for the quick response.

This is what I found about the laptop in the Control Panel:

Processor: AMD Sempron(tm) SI-42 2.2.0 GHz
Installed memory: 3 GB
System type: 64-bit Operating System
Network Adapters:
Atheros AR5007 802.11b/g WiFi Adapter
Microsoft Virtual WiFi Miniport Adapter
NVIDIA nForce 10/100/1000 Mbps Ethernet
Teredo Tunneling Pseudo-Interface

keith1620 said:

Atheros AR5007 802.11b/g WiFi Adapter

While on paper 802.11g gets 54/Mbps speed, the average actual is around 20/Mbps. You can upgrade to a 802.11n adapter and should get the 50/Mbps on 5Ghz.