樓主要不要參考一下這裡 PS:抱歉是英文的

Question: My cheap ps/2-USB green or purple adapter is not auto-detected by Windows plug and play and I can’t get it to work. Where can I download the drivers?
(two typical examples of passive ps/2 to USB adapters)
Answer: The vast majority (95%+) of cheap ps2 to USB adapters are simple passive or pass-through devices that only function to connect the ps/2 wires to the approximate USB wires. These generic green or purple colored adapters do not have custom software drivers. They were intended to only be used with specific keyboards that have built-in logic that can auto-switch between ps/2 and USB modes.

In general, a well-designed converter will use an integrated circuit (IC chip) to actively translate the ps/2 keyboard signal and convert it into a USB keyboard signal. This allows the vintage ps/2 keyboard to be automatically recognized by the operating system as if it were a standard, modern USB keyboard. A well-designed active ps/2 to USB converter will use the built-in operating system drivers for a USB keyboard (for example in Microsoft Windows XP, kbdclass.sys and kbdhid.sys).
http://www.microsoft.com/whdc/device/input/default.mspx

內容大意上跟 darksnow兄 說的一樣