Did it show any message before it boots into Windows (during boot)? before the instant of shutting down did it make a sound like "Beeb" ? you have to make sure that laptop is well cooled by lifting it up a cooling pad or even a big book but don't block the vents and the fan...ensure the fan isn't making any noise while booting up or while working,if it is a new laptop suspect that it is defective and search for the defect and return it..Ensure that ur fan is clean of dust , don't sacrifice the overheating as it could be deadly for the MB, if that shutting down happen when u open a program then it 'd be that program uninstall it and search for an alternative to it ..if u have recently setup a windows copy then check the file system more important u check ur drivers and ensure that it updated and may be it isn't compitable with that copy of windows then u 'll have to change the windows copy,maybe ur MB is just getting tired if u stay on ur laptop for too long..tell me ur laptop model.