As has already been explained, windows disks are rarely sold with any pc or notebook with windows pre-installed.
If the second hand notebook has a working windows os installed, you can clean it up (ie remove unnecessary software, malware and files) and it should be ok for you to use, providing it was genuine to start with.
Another option is to take it to the notebook manufacturer and see how much it will be for a reinstallation. If you are lucky and it is still under warrantee, they may do it for nothing, otherwise they may do it for a fee that is substantially less than buying a new windows disk.
Third option is to buy a new windows disk. As Saab said, you could try OEM or you could invest in a full version. The viability of buying a new OS is dependent on how much you paid for the second hand notebook, although a full version can be used on a different system when you are finished with the first one.