Formatting a PC and reinstalling Windows maybe troublesome for some, but it's much easier and faster if you have a bootable USB installer. For some cases with faulty DVD drives (such as my own), a USB Windows installer is the only option to go.
Creating a bootable USB installer is really easy, when you're getting a USB stick do note the following requirement:
For Windows 7 - minimum 4GB capacity
For Windows XP - minimum 2GB capacity
1) Put the Windows Disc into your DVD drive. If you have an .ISO file and not the physical disc you might want to read here first.
2) Start WinToFlash, click 'Windows Setup Transfer Wizard'.
3) Choose your DVD drive in step 1 and your USB drive (Warning: All contents will be erased, make sure you've selected the correct drive letter!)
4) Transfer process starts in the next screen. Get yourself a coffee for the mean time.
Unless you're using a fairly old model, most PCs support booting from USB by default. Although you might need to configure your Bios settings to boot from USB (refer to manual or seek help) for some cases.