Argos has this Guard Cat for Laptop / PC (with IR sensors) for just £7.49 (was £14.99)
Will be a great stocking filler for under £8. Reserve online and collect in your local store.
This USB Kitty Sentinel is innocent enough looking to have come straight from Grandma's house, but hiding inside its feline exterior are two infrared sensors. When that weirdo from the cubicle next door tries to read your email while you are out to lunch and trips the sensors the little kitty will hiss, meow and sing to scare the intruder away. If a plastic singing cat doesn't keep them away from your desk, I'm not sure what will.
Features : Animated Cat figure | The package contains CD with software, user guide, quick start guide and online support link | Infrared sensor that detects movements from up to 3 feet away (guard mode); program different meows and hisses to alert you of any intruders | Uses VMS technology | Compatible with Windows XP | Plugs into your USB port for power - no batteries required.