When I was installing SQL Server 2005 last weekend, one of the first things I wanted to find out was whether I could run SSRS 2000 and SSRS 2005 on the same computer. Nothing was carved on the Books Online stone for SQL 2005 or 2000 versions, but I presume it is possible to install/manage Reporting Services as a windows services. Further dig out in Books online for SQL Server 2005 (May 2007 I think) and in this case you must have Windows Server 2003 operating system with SQL 2000 edition as a default...(
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Read the complete post at http://sqlserver-qa.net/blogs/tools/archive/2007/10/08/how-to-run-reporting-services-2000-and-2005-side-by-side.aspx