How to download Windows Vista SP1 early
The disc image weighs in at 1.2GB and must be used to perform a clean install of Windows Vista SP1. In other words, you cannot use it to update your system and retain your files and settings. We'd recommend waiting until Microsoft pushes SP1 out through Windows Update if you're not ready to do a clean install.
Microsoft's servers are being hammered at the moment from people attempting to download SP1, so don't expect a speedy download. In fact, if you start downloading today you might not have the full disc image until March anyway.