You know, there's a cheap little bluetooth device out there that might help you.
You plug it in to your USB port and keep the "dongle" in your pocket.
When you walk away from your PC (say 6 feet or so) the computer automatically locks. When you come back within range it automatically unlocks.
I finally found the one I have. Its a PC Defender Wireless Screen Lock. Not sure if they sell them anymore but I've seen plenty of them out there at places like Fry's Electronics, etc. Here's a link to show you what one looks like:
On that note... there are also programs out there such as Blue Lock 1.92 and others which can tie in to the Bluetooth on your phone and perform certain actions (usually lock the screen) when you walk away from your PC (taking your phone with you).
Another solution would be to use your webcam to take periodic screenshots, say every 5min. Then just count the shots that have your face in them. Microsoft has a Power Toy that does this. Also doubles as a security camera. ;-)