This is sorta old news already but I guess Windows 7 activation was hacked within days of Windows 7 official release:
According to Windows enthusiast site Neowin, one of the first to report the crack, a Lenovo disk image of Windows 7 leaked to a Chinese Web site, then moved to English-language domains. Pirates proceeded to retrieve the master OEM key and the OEM activation certificate from the .iso file. Microsoft lets major computer makers like Lenovo, Dell and Hewlett-Packard pre-activate new PCs at the factory to save customers the hassle, and provides OEM master keys for that purpose.
Windows 7 uses an updated activation scheme, dubbed OEM Activation 2.1, which is an updated version of the activation software that first appeared in Windows Vista. The technology, ironically, has been the focus of a Microsoft lawsuit filed last January against a former employee charged with stealing company documents related to the anti-piracy software that computer makers use to lock Windows to their PCs.
My advice? You probably will want to get the official version because a lot of those hacked Windows 7 versions might potentially come with a Trojan horse.