If you’re running Windows 7, but you no longer have a copy of the disk, you might have the option of downloading it straight from Microsoft. There is one catch however, you must be able to verify the product key for your copy of Windows before you download it!
Here is the link: https://www.microsoft.com/en-us/software-download/windows7
If everything is successful, you be given the option to down either the 32 bit version or the 64 bit version. Most likely you will choose the 64 bit option to download since modern computers for quite some time run completely in 64 bits.