Download the corresponding edition of Windows 7 you have a license for from the following link. You can identify the edition on COA sticker attached to your machine, with at bottom or battery compartment (laptop) or top/side if its a desktop computer.
COA Certificate of Authenticity:
What is the Windows Certificate of Authenticity?
• Windows 7 Home Premium (x64) - 64 bit
• Windows 7 Professional (x64) - 64 bit
• Windows 7 Ultimate (x64)* - 64 bit
ImgBurn - In addtion to supporting the creation of CD's from .ISO files,
it supports a wide range of other image file formats, and it's free.
(BIN, CUE, DI, DVD, GI, IMG, MDS, NRG, PDI and ISO)
http://www.imgburn.com/
Note: Always use the slowest burn speed (4x or 2x) if offered a choice.
UPDATE:
For the past 5 years Microsoft made it quite easy for users of the Windows 7 operating system to obtain reinstall media. If you lost your retail installation disc or recovery media; either it was damaged or faulty; you could download a copy of the edition you have a license for from an affiliate website named Digital River. Most recently, Microsoft ended availability of reinstall media which you could download as a .ISO file from Digital River, which is a digital replica of a optical disc.
This was especially handy for persons who could not easily obtain recovery media from the manufacturer, did not want to pay the cost required to obtain it or preferred a clean configuration without the manufacturers bundled software or even a non-functional recovery partition. It was easy to use, all you had to do was reinstall and reactivate using the product key located on the certificate of authenticity and download any appropriate drivers from the manufacturers website.
Since this option is no longer available, what are your options? See article for details: