For me, its after the OS is installed.
I have to go through and add all my favorite apps.
PhotoPaint, ACDSee, Roxio Platinum, Nero Ultra, Adaware, Office, some Dungeon making programs, DC++.
Then I gotta setup Outlook with all my email accounts.
Oh, cant forget all the changes I need to make in Control Panel. Very few things are set up the way I like with a default XP install. Not to mention the service packs from Windows Update. And the SATA drivers, along with the USB drivers for my add-in card.
I need to find the program that lets me customize my Windows disc and reburn it.