On March 18, Microsoft will post to its download site the manual installer for Vista SP1. This will be a big one, several hundred megabytes in size. On the Vista upgrade disk Microsoft sent me, it weighs in at over 400 MB.
If you wait a month or so, Microsoft will deliver SP1 in April via Windows Update. If you've been keeping up with the various patches and fixes for Vista you'll see a much smaller download, as Microsoft will send you only what you need.
Microsoft, Windows Vista, SP1, Vista SP1