I have tried everything from system-config-network to configuring the network files but it is prompting that eth0 is an unknown interface & is unable to bring it up. while restarting the network services by /etc/init.d/network restart it prompts rtl8129 device doesnot seem to b present, delaying initialization. plz suggest sumthing.

If you google for "rtl8129 device does not seem to be present" you can see that there seem to be (or have been) issues with the driver. The issue seems to be fixed now though. Is the system fully updated? If its a new install, is it perhaps an older version? What system/distro/kernel version are you using? When you ask a question, you should try to supply as much information as you can, otherwise no-one can give you specific help with your problem.
Always try googling for the error message - I've solved most of my own problems doing that

well sorry for the incomplete information. i am using RHEL 5. the card details are RTl 8111/8168B PCI Express Gigabit Ethernet(rev03). It is working fine with windows. I have googled it & tried everything from modprobe to lscpi & all, except the last option downloading the driver. Plz help!!!

Spell out your words.

And as andywebsdale said, it was a bug, and has been fixed. Your problem, though, is probably that you're not PAYING for RHEL, and (since you're not), have not received any of the updates/patches, that would make that device work. Your solution is simple: pay for RHEL (which gets you access to the RedHat network, with patches/updates/fixes, and their tech support), or load CentOS (which is nearly identical, but free).

Thank you sir. And i will work on that certainly.