After installing Ubuntu (fiesty fawn), I learnt the hard truth that my LAN Card (supposedly a realtek 8139) is not compatible.
I tried doing $>lspci (which lists all PCI devices), I got this
01:08.0 Ethernet controller: Hangzhou Silan Microelectronics Co., Ltd. Unknown device 2031 (rev 01)
Shit! I was fooled into buying a fake of Realtek card. My LAN card came packed in a box with the name Zebronics technologies. On googling, I learnt that all Zebronics and Intex LAN cards are fakes using the cheap chinese products.
These cards work on Windows XP (after installing some drivers. It is not plug and play.) but not on Linux. , so all I can do now is buy a VIA Rhino or D-Link lan card which should just work with Ubuntu.
Future buyers, beware. I repeat, do not buy Zebronics and Intex lan cards just because they are cheap.
Here are other links from which I got to know about this
UPDATE : see this http://vishalmanohar.wordpress.com/2007/07/07/configuring-hangzhou-silan-on-ubuntu-linux/ on 2 ways to configure the Silan card on Ubuntu linux