windows.old
You may have a folder called Windows.old on your C: drive. This will have all your documents, etc.
Some words from a website:
You can clean install Windows Vista on top of the previous operating system and still have everything available. This is possible via the Windows Vista Setup. The procedure is simple and painless. All you have to do is run the Vista setup while being logged on your current operating system. Choose to Install Now Windows Vista and simply keep clicking next at all the prompts that are presented to you.
After you enter your product key, select Custom (advanced). "The partition you selected might contain files from a previous Windows installation. If it does, these files and folders will be moved to a folder named Windows.old. You will be able to access the information in Windows.old, but you will not be able to use your previous version of Windows," is the message that Setup will display when you will choose to clean install Vista on top of your current operating system.