how to send an email at specific interval of time using c# Asp.net web application

Question

i have a column in my application as "Renewal period" values strating from 1 to 5 years  such that when ever any client get registered with our services he should get an email as a remainder a week before  end of his renewal period.

This thing should occur automatically

kindly help me and iam using sql server 2005 as back end... Thank u