C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Data
or
D:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Data

where ever you have installed it

You could could get the location from the database itslef
open microsoft sql server management studio express
right click on the database name
select properties
select file
and you could see the path 
:)