If you don't have your Windows 7 disc handy — but want to create a custom installation, run Windows from a USB drive, or just do a fresh install — you'll need an ISO file of the disc. You used to be able to download them from a site called Digital River, but those links no longer work. Now Microsoft has a Software Recovery Center where you can download those ISOs for free.
This isn't piracy, of course — you still need a valid Windows licence to download the ISO. If you purchased a retail version of Windows, enter the product key from the package. If you can't find it, use a program like Magical Jelly Bean KeyFinder. Once Microsoft confirms your product key, you can download Windows and use the Windows 7 USB Download Tool to put it on a thumb drive.
If your computer came with Windows, however, it's probably an OEM version, which will not work with Microsoft's new site. Instead, if you want to reinstall Windows without the bloatware, you'll probably need to borrow a disc from someone and use your product key when you resinstall.
Windows 8.1 users have always been able to download ISOs with Microsoft's tool, which you can now get here.