Get SQL server instance installation date I would like to know the date when the sql server installed on the system. 
I did try to look in %programfiles%\Microsoft SQL Server\100\Setup Bootstrap\Log\ directory but there are so many files and folder not sure from where I can get the required information.
You could use the sys.server_principals view to get the date:

SELECT  create_date
FROM    sys.server_principals
WHERE   name = N'NT AUTHORITY\SYSTEM';


Check out server_principals on MSDN.