It depends on how correct you want to be. The second one is the more correct form, because 'whom' should be used for the object of the sentence. The first one is ok, too. It's less 'correct', but it's acceptable in informal English. Though, in fact, the most natural way of saying this wouldn't involve either 'who' or 'whom'. The normal thing would be to say: The boy I'm looking for is near the window. You don't actually need a relative pronoun at all in this sentence.

In spoken English in the US people don't use whom unless they are trying to be very formal/proper. A more natural-sounding way to say this sentice in conversation is "the boy i'm looking for is near the window"

In modern English the most common usage is who in all occasions as whom is considered to be too formal and old fashioned, however it is not incorrect to use whom. You can see more info here http://www.oxforddictionaries.com/words/who-or-whom

You are right, Mohammed.

Both are fine