If you're installing Windows via a boot DVD, and you choose Custom, you have the option to rearrange partitions.  I like use this to have each drive be one big partition.

Windows 7 wants to set aside a 100 MB partition for something-or-other.  I'm sure there's a very good reason for this but I am too lazy to look up the team that owns this space and ask them what it is.

So I'm in the "Where do you want to install Windows?" stage, I've gone into "advanced drive setup", and I've deleted all the partitions.  Fine.  I then create a partition that fills the drive, and I get this popup:

Install Windows
To ensure that all Windows features work correctly, Windows might create additional partitions for system files.
OK | Cancel

After letting Windows finish installing, I jump into diskpart.exe and sure enough, Windows has created an additional partition.  A small one, to be sure... but an additional partition (horrors!)

Not being one to let Windows push me around, I decided to experiment, and came up with the following dance to allow me to just have one big partition thankyouverymuch:

  1. Boot from the Windows DVD
  2. Choose Custom (advanced) as opposed to Upgrade
  3. Go into Drive options (advanced)
  4. Delete all partitions on the drive
  5. Create a new partition - you will get the prompt above
  6. Click OK
  7. There will now be two partitions - a small (System) one and a large (Primary) one.
  8. Delete the large one.
  9. Extend the new one to fill the drive.
  10. Install Windows.
  11. Open Windows Explorer
  12. Right-click the C: drive | Properties
  13. Delete the "System Reserved" partition name
Et voilà, Windows installs perfectly happily on the single partition (confirmed with diskpart.exe post-installation.)