Try this on the computer.
Uncheck the Network Card's Power Saving (I.e., do not let the card save power).
There might be some variations on where the Power saving setting is.
Here are few examples, YMMV, look around in your systems.
Example,
http://www.ezlan.net/example/powersave.jpg
Example,
http://www.ezlan.net/Win7/power_sav_wireless..jpg
Example,
http://www.ezlan.net/Win7/power_save_win7.jpg
Example,
http://www.ezlan.net/Win7/adv_power-sav.jpg
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Still a problem?
Check the Router's support, and make sure that your using the latest firmware for the Router.
Check the Computer's manufacturer support site and make sure that you are using the latest Drivers.
Download the drivers and unpack them to a folder of your choice.
In the Device manger, right click on the adapter.
Click on Update Drivers, and choose the Browse my computer for drivers,
Choose let me pick from a list, Click on Have a Disk and point to the folder with the Drivers.