hi, thanks for the reply.
my OS is windows 8 pro 64bit and my problem happened after installing windows 8.
i checked my wireless adapter from devise manager and it is Dell Wireless 1702 802.11b/g/n. but see here www.dell.com/.../pd it says wireless adapter is Intel® Centrino® Wireless-N 1030 (1x2 bgn + Bluetooth) for this model.
i booted the computer in 'Safe mode with networking' and the system exhibits same behavior.
i downloaded and installed the latest driver for windows 8 from dell website and still the problem remains.
please help me to solve this. :(