Like ss202sl said, use a dvd with an OS on it if you have one, or the one that came with the laptop OR if it had Windows on it just download the OS from one of Microsoft repositories and burn to a dvd - cant remember where it is by a simple Google search will probably find it. I did this for a Windows 7 Laptop that had a wiped HDD in it.
You just need to install the correct OS that your laptop has a licence for. You can find this and the OS licence number on a sticker somewhere on the machine.