The Problem: When I play Half Life 2 (or CS, CS:CZ, HL 1), the player
moves on the screen forward, backwards, left or right without me
pressing any of the direction keys (W S A D).

I can play the game anywhere from a minute to five minutes before the
problem happens.  It doesn't seem to happen at any point in
particular.

My keyboard and mouse work just fine outside of these games and in all
other circumstances.

I have the following config:

Half Life 2
AMD Athlon 3000+
2GB RAM DDR 400
300GB SATA I HD
Nvidia GeForce 7950GT (512MB/PCI express x16)
Windows Vista Home Premium x64 (Legit)
Biostar NF4 4X-A7 mobo (NVIDIA nForce4 4X chipset)

This is what I've done to troubleshoot:

Disable all background apps.
Run AV and antispyware programs and nothing was found.
Updated video driver, chipset driver, and AGP driver to the latest.
Remove Steam and HL:2 and reinstall both, with updates.
Check for device conflicts; none found.