Make sure you have the right driver for the card. Check Control Panel, System, Hardware, Device Manager to see if its listed or yellow flagged as faulty.
If this is a PCI adapter I would also disable any spare ports in the computer's BIOS (COM, Parallel Printer Port, Game/Midi)
Try reloading the driver.
If the card then works (finds wifi networks) but problems persist I would try turning off wireless security temporarily in the router and check that wireless mode is correct.