Let's be clear: we're talking about importing a laptop for resale or as a gift (hence the box), but avoiding payment of required customs duties.

A computer for the passenger's personal use is not likely to be in pristine condition and packed in a box such that it can potentially be sold. Customs officials are not stupid.

On the other hand, most bags are not opened, and I don't know if X-rays will see the box. (On my most recent trip to CCU they didn't even X-ray my hand luggage.)

To be legal, it should be declared. What the quoted rules are saying is that apart from the used laptop for personal use, a dutiable (new) computer can be imported as part of the duty-free allowance. The allowance for people of Indian origin is Rs. 25, 000.