You definitely need a product code, which should be with the CD (or at least the case it came in). Sometimes the product code sticker is put onto the case itself. If you have the product code, you can download Windows (it will ask for a code in all likelihood before letting you download it), then you can create a bootable USB using Rufus. But this solution only works if you have the product code, since you will need it to do the install.
Otherwise, you either have four options:
1. Leave it as it is
2. Find the CD
3. Buy a new copy of Windows
4. Switch to Linux
5. Find a way to extract the product key from your existing install. I've never done it, but I believe there are programs to do it.