With KB 943198, USB-to-Serial adapters are no longer crippled with slow transfer rates

SYMPTOMS
When you use a USB-to-Serial converter to connect a device to a Windows XP-based computer, the data transfer from the computer to the device is slower than expected.

CAUSE
This issue occurs because the Usbser.sys driver does not follow the USB specification. When the computer transfers data to a mobile device, the transfer process is based on the Usbser.sys driver. The Usbser.sys driver does not send zero-length packets when the total transfer size is an integer multiple of the packet size. However, zero-length packets are required by the USB specification so that the device can recognize when the packet transfer is finished.

Thankfully, this is now a public download and no longer requires requesting the hotfix. You can download the hotfix here.

Or, you could install XP SP3 RC2, which includes this fix already.