I need to write a tool that will run a recurring task on a user configurable schedule. I'll write it in C# 3.5 and it will run on XP, Windows 7, or Windows Server 2008. The tasks take about 20 minutes to complete. The users will probably want to set up several configurations: e.g, daily, weekly, and monthly cycles. Using Task Scheduler is not an option.

The user will schedule recurrences through an interface similar to Outlook's recurring appointment dialog. Once they set up the schedule they will start it up and it should sit in the system tray and kick off its tasks at the appointed times, then send mail to indicate it has finished.

What is the best way to write this so that it doesn't eat up resources, lock up the host, or otherwise misbehave?