With the advent of cheap high capacity USB flash drives our reliance on optical drives is continuously decreasing.
PCs with malfunctioning or non existant optical drives face a dilemma while having to reinstall operating system which are most commonly in the form of bootable DVDs.
Here is a step by step guide to installing windows 7 from a USB flash drive.
This article assumes that you have the operating system files stored on your HDD. If the OS is in .iso format, extract it using WinRAR or 7zip (or any other utility)