i would like to install MS-SQL 7.0 and 2000 in my PC. I

Akansha,

You can not have SQL Server 7 and 2000's "default instance" running on the same machine.
If you want SQL Server 7 and 2000 running on the same machine then you will have to
install sql server 7 first and then named instance of SQL 2000.

You will get ample of information in SQL Server 2000 Books online on following topics and
its subtopics.

"Running SQL Server 7.0 Along with a Named Instance of SQL Server 2000"
"How to install a named instance of SQL Server 2000 (Setup)"
"Working with Three Versions of SQL Server"
"Setup and Installation FAQ"
"Setup Type: Typical, Minimum, or Custom"

latest books online download at

http://www.microsoft.com/sql/techinfo/productdoc/2000/books.asp
http://msdn.microsoft.com/library/default.asp?URL=/library/psdk/sql/portal_7ap1.htm


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-Vishal