We use host monitor for network and server monitoring and also for this purpose.
http://www.ks-soft.net/hostmon.eng/I have also used WinBatch to do similar things. You could have it running and constantly monitoring the folder or use the scheduler to check at regular intervals.
Back in the windows 95 days we used an excell spreadsheet running VB code to do this. It was set to check every minute from 7am - 6pm and then every 15 minutes at other times.
Freeware program to do it
http://www.snapfiles.com/get/automailer.html I have no experience with it but it looks like it will do what you want.