Need Script Coded to search certain websites for certain terms/keywords

I need a small, simple script or desktop application coded. I need the script to search certain websites/blogs/forums, etc that I define for certain keywords/terms/phrases that I define. The script needs to do this 24 hours a day without any intervention from me. In other words I don't want to have to keep putting in the terms and starting the script.. I want to set and forget. But if I want, I need the script/software to be stopped and new data re-entered from me. The script should run on crons if it is a script, unless there is a better way. I need to enter the dates to check forum/blog posts for certain keywords/phrases. And I need the software/script to pull the entire posts that have urls in them. I also need the script to alert me via email when the keyword/phrase/term is found. In a nutshell, I need the script to act much like Google Alerts, but I don't want to wait until the keyword is indexed to get alerted, I want the script to actually spider the site for updated terms/keywords/phrases. The script should know what it has already sent me and not send it to me again. Maybe it could grab the date/time of the post(blog or forum) and only send updated info. I hope I've describe this good enough, if not, PM me and we'll discuss it further.