the problem with the at command is that you can apparently schedule command only by date:

For example:
To back up the hard drive of the Marketing server to a tape drive at midnight every five days, create a batch program (Archive.cmd) containing the backup commands. Then schedule the batch program to run by typing:

at \\marketing 00:00 /every:5,10,15,20,25,30 archive

I don't know how to schedule command every 15 minutes