How do I detect user proxmity to lock PC.?
RFID sounds a good solution. My cat flap uses it and despite my reservations, it's still going 3 months down the line on the same pair of AA batteries.

OK my cats are chipped of course BUT that is what RFID is. What powers it? A local magnetic field around where the cat enters or where the person sits. Range depends on the magnetic power that can be pushed to the RFID tag. More power means bigger transmission power means bigger distance.

Cat flaps or folk sitting/leaving their chair are about the same order of magnitude.

What do you need? A small micro with an RF transmitter that is powered from a locally generated magnetic field. The field induces voltage in a small coil in the tag. This coil is resonant tuned to the frequency and suddenly several volts of DC can be conjured out of the air. Enough to power a small micro and transmitter. The transmitter transmits an ID every second or maybe every ten seconds and a local receiver tied to the USB port will send the appropriate commands should the user go out of range or come back into range.

Perfectly feasible and sounds a cool idea.