The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
A laptop computer is essentially a large filing cabinet and many people keep their financial records, personal communications, business data, entertainment, etc.. on thier computers. The "papers" referenced in the fourth amendment are now digital files, and of course the government does not recognize this. This ruling also applies to iPods, phones, and basically anything with digital memory.