after installing windows as single boot
i installed explicitly "atheros l2 fast ethernet 10/100base-t controller"
driver in system to access local area connection...
i googled a little about it.
temprory working solution for my system is following(i need better one):
"As ubuntu is saving some setting in hardware which windows can't understand.
So prior to boot windows restore bios factory settings and then boot windows..
There is no need for this between 2 consecutive windows boot
But its mandatory after booting ubuntu once..