use webservice

we can achieve this either in the windows application with
the windows schedule task or sql server 2005.

How with Webservices and how with windows schedular and
sqlserver2005 plz?

There are two ways you can achieve this.

1> When time is fixed for scheduler

--> Create  a windows application which sends mail to the 
clients when its executed
-->add it to windows scheduler(schedule Task) at specified 
time

2> When you want cofigurable time for scheduler


--> Create  a windows application which sends mail to the 
clients when its executed
--> create a windows service add a timer to the service and 
set interval to 2 mins suppose.

-->On the timer elapsed event add the code for executing 
the above executable 

-->before executing  you can check that if the current time 
is equal to the time specified by user or not..(you can get 
it from database or .config file also)

-->if current time = specified time or +2-2 minutes ( as we 
have specified interval of 2 minutes so its possible we may 
not get exact match so matching range of + or - 2 minutes)


-->install it for scheduler using installutil.. statrt 
service

So above scheduler will check after every two minutes for 
the matching time and if time matches it would execute the 
exe for sending mails. otherwise it would keep on checking 
for matching time.

Thank u very much
  But in the case of Web Apps how to approach this,how to 
send mails to several of my clients with out click event or 
with out execution like www.123greetings.com

One more way is that, you can create a windows service and 
install it on your server. 

In windows services you can place a timer so you can 
configure a time for execution. 
:)