What I've done in the past is set Windows up on any computer capable of running it, install all the necessary software, patches and so forth, then use Sysprep (on the Windows install CD in deploy.cab) to "reseal" the installation. If you're then going to copy it to a computer with different hardware, you choose the mini-setup option in Sysprep as well, which forces Windows to go through its setup routine, including rescanning and installing hardware, on the next boot (if you're copying to identical hardware you just need the regular reseal, which just regenerates the SIDs, allows you to enter a different license code, user names, etc). Then you can use Norton Ghost to image the whole partition and turn it into a bootable CD/DVD that can be used as a restore disc. The image files are stored on the CD(s) or DVD(s), and therefore don't take up hard drive space as I have seen on many manufacturer's setups (ie. they include a clean boot partition image on a secondary partition, and a CD or floppy to boot into Ghost or similar and restore the image - which is bugger-all use if the image file has been deleted, or the hard drive dies with the clean image file on it).
As for what Dell et al do, they create customized version of Windows to supply with the computer, which installs other elements, or modified elements (like setup screens with the company logo on). It's not an area I've looked into in any great detail, but apparently this site...
http://unattended.msfn.org/
...has some great guides on how to streamline everything from service packs and drivers to entire applications into a customized installation CD.