If you want to evaluate or test a Microsoft product before purchasing it then the best way to do this is by downloading the product from the Microsoft Trial Software Center.  You can find trial software for Windows, Office, Developers Tools, Servers, Games & Xbox and Finance for both Business and Consumers. 

At the time of writing this article here was the list of trial software available.   If the link below no longer works check the website for updated offers.

Windows
  Forefront Client Security Trial Software
  OneCare Trial Software
  Windows Home Server Evaluation Kit
  Windows Server 2008 Trial Software
Office
  Forefront Security for SharePoint Trial Software
  Office Accounting Professional 2008 Trial Software
  Office Communicator 2007 Trial Software
  Office Enterprise 2007 Trial Version
  Office Groove 2007 Trial Version
  Office Home and Student 2007 Trial Version
  Office Live Meeting Trial Service
  Office OneNote 2007 Trial Version
  Office Professional 2007 Trial Version
  Office Project Professional 2007 Trial Software
  Office Project Server 2007 Trial Software
  Office Project Standard 2007 Trial Version
  Office Publisher 2007 Trial Version
  Office SharePoint Server 2007 Trial Version
  Office Small Business 2007 Trial Version
  Office Standard 2007 Trial Version
  Office Visio Professional 2007 Trial Version
  Office Visio Standard 2007 Trial Version
Servers
  BizTalk Server 2006 R2 Trial Software
  Commerce Server 2007 Evaluation Software
  Exchange Server 2007 Trial Software
  Forefront Client Security Trial Software
  Forefront Security for Exchange Server Trial Software
Office Project Server 2007 Trial Software
Office SharePoint Server 2007 Trial Version
  Small Business Server 2003 R2 Trial Software
SQL Server 2005 Enterprise Evaluation Edition
System Center Configuration Manager 2007 Trial Software
  System Center Data Protection Manager 2007 Trial Software
  System Center Essentials 2007 Trial Software with Service Pack 1
  System Center Operations Manager 2007 Trial Software
  System Center Virtual Machine Manager 2007 Trial Software
  Windows Home Server Evaluation Kit
  Windows Server 2008 Trial Software
Developer Tools
  Expression Blend 2 Trial Software
  Expression Design 2 Trial Software
  Expression Encoder 2 Trial Software
  Expression Media 2 Trial Software
  Expression Studio 2 Trial Software
  Expression Web 2 Trial Software
Forefront Client Security Trial Software
SQL Server 2005 Enterprise Evaluation Edition
  System Center Essentials 2007 Trial Software with Service Pack 1
Visual Studio 2008 Professional Edition Trial Software
Visual Studio Team System 2008 Team Foundation Server Trial Software
Visual Studio Team System 2008 Team Suite Trial Software
Visual Studio Team System 2008 Test Load Agent Trial Software
Games & Xbox
Age of Empires II Trial Software
  Age of Empires III : The WarChiefs Trial Software
Age of Empires III Trial Software
Age of Empires III: The Asian Dynasties Trial Software
  Flight Simulator X Trial Software
  Halo for Windows Trial Software
  Rise of Nations: Rise of Legends Trial Software
  Zoo Tycoon 2 Trial Software
  Zoo Tycoon 2: Extinct Animals Trial Software
  Zoo Tycoon 2: Marine Mania Trial Software
Home and Entertainment
  MapPoint 2009 North America Trial Software
  Math 3.0 Trial Software
  Office Publisher 2007 Trial Version
Finance
  Money Essentials Trial Software
  Money Plus Deluxe Trial Software
  Money Plus Home & Business Trial Software