There have been lockup issues with the drivers for USB to serial adapters built with the Prolific chips for quite a while and most folks have changed to FTDI products which work fine for the 4170.
However Prolific has now fixed the problem. See www.prolific.com.tw/Eng/downloads.asp?ID=31 for the new drivers. You need Driver PL2303 installer ver 1.10. I have heard that the old driver needs to be manually removed before adding the new one but I can't verify that since I had removed the old Prolific driver long ago. Nevertheless the new driver works great with the 4170 for Prolific USB to serial adapters. I just did 50k recycle scans without any lockups. I tested on WinXP Pro and Win7.
Just FYI for those who didn't throw out their Prolific USB to serial adapters.