If you already do not have USB disc, you can buy it from market or online (or borrow from your family member or friend, if you like so). If you already have USB drive or memory card, use it. Anyhow, if ISO file size exceeds 4GB, you will definitely need at least 8GB USB disc. Some ISO files (All in one pack or 32 bit and 64 bit in single ISO) have large size than 4GB. Choose either of these but this media should have at least 4GB storage space. Step 2: You need a USB flash drive or a memory card. Use Rufus to create Windows bootable USB from ISO image

Windows XP ISO (including Service Pack 3).Download Windows 7 ISO (bootable disc images).Step 1 is Windows ISO file, you can download your desired ISO from Windowstan. A USB flash drive or SD/micro SD card (4GB or 8GB).Windows ISO file (available on Windowstan).If you want to install Windows 10, Windows 7 or any other version and do not want to buy or use DVD disc due to any reason, you need following things to install Windows without DVD: Rufus is popular for creating Windows installation media which can be a USB drive or an SD card or micro SD card. Rufus is a small software that can load the ISO file and create bootable USB drive for installing any operating system.

Rufus software