There is not much you can set on the pc side to affect the speed. In general the only things you can set will make the connection worse. This device will only connect on 2.4g and you don't want to force it to connect using the older encoding like 802.11g.
You see the speed it connects to the router with in one of the status pages for the nic. This is not a actual speed though it is more the data encoding method it is using.
The nic and the router negotitate the best possible options they think will work. You need to make sure you have the latest drivers for your nic but that is going to be the best you can really do with any software.
Those tiny wifi nic cards are designed for portable devices. They fit almost completely inside the machine. On a laptop this is less a issue since it is made mostly of plastic. A desktop is made mostly of metal and you are putting your antennas almost inside a metal box. Those tiny devices also have low power radio to save battery power which affects its performance even though a desktop does not use batteries.
Try a USB extension cable and see if you get better signals.
I wrote all this and come back to find I never did post