You have a couple of options when reinstalling Windows Vista, and in your situation you did one of two things. You either inserted the DVD while the computer was on, and performed an "In-place upgrade". Or, you booted from the DVD, and performed a "Clean install" to the same hard drive without first formating it.
If your desktop is exactly the same as it was before you did the reinstall you performed an in-place upgrade. Should you have a C:\Windows.old directory you performed a clean install without format. However, it's a simple process to remedy.
You can do a quick search for 'WindowsReinsatll', and you'll find several walkthroughs some even with pictures. But, what you'll need to do is boot from your DVD, and then perform the reinstallation. If you can't find a walkthrough, or have any questions please feel free to ask. However, keep in mind the information is "as-is", and a reinstallation will result in data loss so you'll want to backup any irreplaceable data before proceeding.
How do I reformat my hard drive and reinstall vista?? I inserted the dvd thinking it would wipe my hard drive clean, but after reinstalling, all of my old stuff is still on the drive. I googled, and thought that everything I read said Vista would automatically wipe it clean, but guess not........