.net – Design for VB.NET scheduler application

I wish to develop an application in VB.NET to provide to following functionality and hope you can give me some pointers on which direction to take.

I need some kind of “server” type component which sits in the background monitoring request from users and performing various task. (this component can be install locally or centrally)
The users submit an instruction to the “server” to perform a certain task at a designated date and time. (or perform the task straight away)

The “server” would perform the task at the desired date and time and inform the user the result of the task.

I have thought of using a central database to which the user writes the instructions. The “server” could read from the database to obtain the instructions, and write the result back to the database.

I want a fast reaction to the instructions, so the “server” must poll the database every few seconds; I fear this may be detrimental to performance. Also how do I get the server to perform the task at the desired time?

Again checking all outstanding tasks against the current time is not very efficient, so I thought about utilising the Windows Scheduler, but I am not sure of the best way of integrating this functionality.

I would be grateful for any ideas, pointers or suggestions.