At this time last year when I left the electronic security industry there was only 1 software suite that had ever been accepted as reliable evidence to secure a conviction in a British Court.This software was developed in Australia.
This is currently used by the police and I'm sure if you contact your local Crime Prevention Officer they can give
Needless to say it's far from free and is intended for decent cameras using CCD detection which can give an adequate picture at night using infra-red.
Cheap webcam's use CMOS technology and whilst they may give a warning if your around in good light, will be useless in the dark and will not have the range of CCD.
Having said all that try this free suite (for XP)