http://support.microsoft.com/default.aspx/kb/932616/en-us

General information about how to install Windows Vista

Windows XP Service Pack 2 (SP2)

How to determine whether your computer is running a 32-bit version or a 64-bit version of Windows XP

Installation choices for Windows XP Home and Windows XP Home N Edition

Installation choices for Windows XP Professional and Windows XP Professional N Editions

Installation choices for Windows XP Tablet Edition

Installation choices for Windows XP Media Center Edition

Installation choices for Windows 2000

Upgrade paths between versions of Windows Vista

INTRODUCTION

This article discusses the installation choices for consumer versions of Windows Vista. This article does not include information about Windows Vista Starter, about Windows Vista Enterprise, or about versions of Windows Vista that you install by using Microsoft Volume Licensing.
The method that you use to install Windows Vista depends on the answers to the following questions:

Definitions

Upgrade to Windows Vista An upgrade to Windows Vista preserves the currently installed personal files, settings, and programs. You can upgrade only certain versions of Windows to certain versions of Windows Vista.

Custom installation (clean installation) of Windows Vista A custom installation or clean installation of Windows Vista does not preserve the currently installed personal files, settings, and programs. Windows Vista is installed without third-party programs. You can perform a custom installation of Windows Vista by using either an upgrade license or a full product license. However, if you own an upgrade license, you must start the installation in the current version of Windows. At the installation choice menu, select Custom to perform this action.

The upgrade version of Windows Vista An upgrade version of Windows Vista is a license that lets you install Windows Vista if you already own a compliant, licensed version of Windows. You can perform either an upgrade installation or a custom installation of Windows Vista by using an upgrade license. However, you must start the Windows Vista installation in the compliant version of Windows.

32-bit operating systems Windows comes in both 32-bit and 64-bit versions. This article discusses upgrade options when you are running a 32-bit operating system and you are trying to install a 32-bit version of Windows Vista.

N versions of Windows Vista N versions of Windows XP and of Windows Vista do not include Windows Media Center. N versions are designed for customers in Europe. You cannot upgrade an N version of Windows. Instead, you must perform a custom installation.

The following table shows the upgrade paths that you can use to upgrade to another version of Windows Vista.