Simple Software Restriction Policy is a security add-on for Microsoft Windows, published by IWR Consultancy.
Its purpose is to make it considerably harder for unwanted or potentially harmful software to get itself launched on the computer. It achieves this not through the file signatures used by antivirus programs, but through identifying the places on the local disks in which legitimate software typically resides, eg 'Program Files' as opposed to places where executables have no right to be, for example Documents folders. Since this form of protection is not dependent on being able to recognise a known and catalogued threat, it is potentially much more effective against new strains of malware.
It is not an antivirus package or 'internet security suite' and can in most cases be run alongside such software without conflicts.
One of the nice features is its low visibility during normal computer use. It's possible to go through a whole day's worth of creating documents, playing media or adjusting settings without seeing a single message from the policy control applet. If you need to do things which the policy would block such as installing software, then a single Unlock command gives you 30 minutes in which to complete your task without any further nags. Need more time? Just Unlock again before the time is up.
Bear in mind that this is a system-level utility, and incorrect configuration or use may cause trouble with your computer. That said, there have been many thousands of downloads to date, and very few reports of problems.
Please study the manual pages on this site before attempting to modify the default configuration.