Where Can I download Windows 7 ISO Legally?

Windows 7 Professional x64 (64-bit)

Windows 7 Professional x86 (32-bit)

Windows 7 Home Premium x64 (64-bit)

Windows 7 Home Premium x86 (32-bit)

Windows 7 N Version Direct Download Links [Windows 7 N English for Europe – No Windows Media Player (WMP12)]

Windows 7 Home Premium N x86 (32-bit): http://msft-dnl.digitalrivercontent.net/msvista/pub/X16-13457/X16-13457.iso
Windows 7 Home Premium N x64 (64-bit): http://msft-dnl.digitalrivercontent.net/msvista/pub/X16-13459/X16-13459.iso
Windows 7 Professional N x86 (32-bit): http://msft-dnl.digitalrivercontent.net/msvista/pub/X16-13548/X16-13548.iso
Windows 7 Professional N x64 (64-bit): http://msft-dnl.digitalrivercontent.net/msvista/pub/X16-13550/X16-13550.iso

Source

Yes it is legal as long as you have a legitimate key.

These download as an .ISO file, which is used to create an install DVD using most DVD/CD burning software.

Service Pack 1 Update

Windows 7 Ultimate SP1-U ISO

English x86: X17-59463.iso

English x64: X17-59465.iso

French x86: X17-59477.iso

French x64: X17-59479.iso

Spanish x86: X17-58877.iso

Spanish x64: X17-58879.iso

Windows 7 Professional SP1-U ISO

English x86: X17-59183.iso

English x64: X17-59186.iso

German x86: X17-59886.iso

German x64: X17-59885.iso

French x86: X17-59195.iso

French x64: X17-59197.iso

Spanish x86: X17-58866.iso

Spanish x64: X17-58868.iso

Italian x86: X17-59212.iso

Italian x64: X17-59215.iso

Portuguese x86: X17-59246.iso

Portuguese x64: X17-59247.iso

Swedish x86: X17-59271.iso

Swedish x64: X17-59273.iso

Norwegian x86: X17-59229.iso

Norwegian x64: X17-59231.iso

Finnish x86: X17-59192.iso

Finnish x64: X17-59194.iso

Dutch x86: X17-59233.iso

Dutch x64: X17-59236.iso

Danish x86: X17-59891.iso

Danish x64: X17-59883.iso

Traditional Chinese x86: X17-59295.iso

Traditional Chinese x64: X17-59297.iso

Korea x86: X17-59299.iso

Korea x64: X17-59300.iso

Windows 7 Professional N SP1-U ISO (Note: N editions come without media components)

English x86: X17-59335.iso

English x64: X17-59337.iso

German x86: X17-59323.iso

German x64: X17-59327.iso

French x86: X17-59348.iso

French x64: X17-59351.iso

Spanish x86: X17-58871.iso

Spanish x64: X17-58874.iso

Italian x86: X17-59364.iso

Italian x64: X17-59366.iso

Portuguese x86: X17-59395.iso

Portuguese x64: X17-59398.iso

Swedish x86: X17-59418.iso

Swedish x64: X17-59424.iso

Norwegian x86: X17-59380.iso

Norwegian x64: X17-59383.iso

Finnish x86: X17-59344.iso

Finnish x64: X17-59346.iso

Dutch x86: X17-59384.iso

Dutch x64: X17-59387.iso

Danish x86: X17-59318.iso

Danish x64: X17-59320.iso

Windows 7 Home Premium SP1-U ISO

English x86: X17-58996.iso

English x64: X17-58997.iso

Spanish x86: X17-58857.iso

Spanish x64: X17-58859.iso

French x86: X17-59007.iso

French x64: X17-59009.iso

Source