Office comes pre-installed on most HP computers (indeed, most Windows 7 prefab computers). It runs in demo mode for 60 days and then you have to buy it to use it longer.You do have to get Office separately. It does not come with Windows or with the PC (unless you ordered Office with it).
Unless we're talking about your work computer (and work requires it), I highly recommend using anything but Office. There are a couple of equivalent programs that are much lighter weight and typically don't cause you to download security patches on the second Tuesday of every month.