From Evarest Schoofs:
ProcAlert is kind of a firewall. However, unlike other "normal" firewalls, ProcAlert is one which will secure the computer from the inside out.
Whenever a new program is started (whether it'd been started by you or by itself), ProcAlert will alert you and ask you whether you want the program to keep running or not.
This way you can easily terminate any malicious virusses, trojan horses or any other malware!
An easy-to-use interface will make it possible to manage which programs can start and which not. Along with this, ProcAlert will hand you all information about the programs that are running on your computer.
ProcAlert also contains a very powerful feature: you can totally lockdown your computer, what basically means that NO program can be opened anymore, until you turn off the lockdown. This way, you can prevent virusses from starting when you're away, or unauthorized access to your computer.