12.04 dual boot with Windows XP. No network connectivity on Windows.
 

ubuntu 12.04 dual boot with Windows XP. No network connectivity on Windows.
Showing network cable is unpluged.
Driver installed in XP is "atheros l2 fast ethernet 10/100base-t controller".
Internet working fine while booting in ubuntu.

before the install of ubuntu did XP have a working driver for the NIC?

@lleb

yes
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.. :( "

Since your linux system is working can you post output of lspci & also tell us the make & model of computer?