Insert the SD card into your SD card reader and check which drive letter was assigned. You can easily see the drive letter (for example G:) by looking in the left column of Windows Explorer. You can use the SD Card slot (if you have one) or a cheap SD adaptor in a USB port.
Download the Win32DiskImager utility from the here(it is also a zip file); you can run this from a USB drive.
Extract the executable from the zip file and run the Win32DiskImager utility; you may need to run the utility as administrator. Right-click on the file, and select Run as administrator.
Select the image file you extracted above.
Select the drive letter of the SD card in the device box. Be careful to select the correct drive; if you get the wrong one you can destroy your data on the computer's hard disk! If you are using an SD card slot in your computer and can't see the drive in the Win32DiskImager window, try using a cheap SD adaptor in a USB port.