Windows 7 Installation issue with USB

I am using a windows 7 operating system, now I want a fresh window to be installed on my PC. My PC doesn’t have DVD Drive, so I want to install new windows using USB. But I am facing some problems while installing new windows.

I have researched a lot about windows installation using a USB. So I come to know that I need a bootable USB of 8 GB (minimum). But I am having a problem with booting with my USB. It is not booting properly. I need help as my old window is slow due to too many programs and games.