gaurav,
i doubt if anyone can answer your question.. See the thing is, that a computer can be hacked in inumerable ways. IP attacks, password cracks , DOS, trojan attack, application attack etc etc.. each of these topics bny themselves are huge and the scenario keeps changing every day..
Basically if you are looking all out protection .. dont connect to the net .. ever!
but otherwise.. you'll have to devise a security policy at many levels. this will include your password policies, internet fireewalling, application restrictions. user / group privileges restrictions.
But be warned... any networked computer is never fulkly secure ever..
cheers,
Ze