SharePoint guru Andrew Connell does it again with a great MSDN article that shows you how to create custom timer jobs in WSS 3.0.  Timer jobs are a great way to schedule long-running or periodic tasks in such a way that they are automatically coordinated with the server farm topology and can therefore take advantage of load balancing or can be directed to specific servers that are running the services they require.   Read about it on the MSDN site at http://msdn2.microsoft.com/en-us/library/cc406686.aspx.