I have personally used the ISO's from HERE:
With OEM (Dell, HP, Gateway & emachine) and "commercial" and "System Builder" CoA/product codes
These ISO are "uncracked" and REQUIRE a valid CoA/Product code to work
As microsoft policy is that the CoA/product code is what you are buying from them
they are "legal" per repeatedly stated M$ policy.
my personal physical disc for the system I am using to post this message
(Win7 pro x64) was damaged by an optical drive that decided to close by
itself as I was setting the disc on the drive tray and HP for their own
persnickety reasons refused to send me another physical disc.
IF the computer you are working on is a Dell and the computer has a
Coa Label for "Dell OEM" windows 7 I strongly recommend you aquire a Dell windows7 reinstallation disc.
Using a Dell reinstaller on a dell computer makes things go much more "smoothly" when a reinstallation is required.
On a USED computer I'd recommend a clean install
On ANY brand computer DO NOT enter your CoA when prompted to early in the installation process (this page can be skipped). if you don't skip this page quite often a VALID CoA will be ignored and in some cases you
block the typical 30day activation period
Enter your CoA only AFTER you have ALL device drivers installed
and have verified that everything works.
Then when prompted choose click the "Activate now" reminder.
IF the activation doesn't work automatically (I've found that even with genuine retail installers it doesn't always work) don't panic
There is a button on the page to Activate via their automated phone system, choose that option and I've NEVER had that activation method fail.
For a tutorial on Windows7 activation (a recommended read for anyone):