Redmond has updated the System-Locked Pre-installation (SLP) to version 2.1, which supports Windows 7 and Windows Server 2008 R2 – and provides backward-compatibility for Vista and Server 2008. The SLP procedure is used to pre-activate Windows for mass distribution by OEMs. Its latest version requires the BIOS SLIC table be updated, as it contains a new Windows marker. The trick is to mod a system's BIOS to include SLIC 2.1 – which can be done via hard (physical) or soft (emulation) mods.
The original post gets more in-depth, and contains much more information. Naturally, the crack can already be found on various torrent sites. I’m not about to preach right from wrong here, as you won’t listen anyway. That said, if you’re going to download Windows 7 from an unofficial source, at least verify your copy.