Whenever My wireless reaches excellent,the internet will cut off.the name of the wireless card is D-Link

it is connected to another pc.it is a usb wireless card.i'm running win xp pro

The first thing I would try is go to the maker's website and download the latest driver for it. Have you already tried that?

you seem to suggest an ad-hoc connection between two system and the
internet connection of the other system gets broken; eg
ISP-->sys-a <---wireless --> sys-b
Is this your setup?

Yup,my system is connected like that.