Just noticed the other day that windows update (xp) had found new ralink drivers for my wireless card - ralink chips are in several budget PCI-cards like edimax, and the new drivers seemed to improve the latency immediately.
I think ralink actually released the drivers late last year, but they've only just appeared through the windows update system, which didn't need a restart. So if you have a cheap wireless card it's worth having a look on the hardware sub-section of windows update