<p>How can I write a scheduler application in C# .NET?</p>

It all depends on your requirements:
If you have access to a database you use a table as a queue and a service to poll the queue at regular intervals.
If your application is client only (CLI) you can use the system scheduler ("Scheduled Tasks").
Lastly, if your application is only in a database (using the CLR in SQL Server 2005 for example) then you can create a SQL Server job to schedule it.