So you upgraded from XP to Vista or from Windows 2000 to Vista and you have had a bad experience. You want your old Windows back.
Well all is not lost. The following are the steps taken to restore your Old Windows...
Step 1: Determine whether there is a Windows.OLD folder and whether there is sufficient free space on the Windows hard disk
Step 2: Start the Windows Recovery Environment
Step 3: Rename the Windows Vista folders
Step 4: Copy the contents or move the contents of the Windows.OLD folder
Step 5: Restore the boot sector for the previous Windows installation
Step 6: Restore the Boot.ini file for the previous Windows installation of Windows XP or of Windows 2000
Step 7: Close the Command Prompt window, and then click Restart.
Microsoft have released a Knowledgebase Article which describes all of these steps in detail.
How to restore a computer to a previous Windows installation after you install Windows Vista