For purposes of sharing, here is what I did to finally successfully Install the Prolific Driver on my W8.1 64bit machine. The solution was getting the correct "SER2PL64.SYS" file. Thanks to those here, a Microsoft MVP, and Prolific for their patient assistance.
On my windows8 64b computer I started by installing the windows 8 driver PL2303_Prolific_DriverInstaller_v1.11.0.zip It's on the http://www.prolific.com.tw/US/CustomerLogin.aspx website (use guest/guest for user and password).
(This direct prolific driver download link might work too) http://www.prolific.com.tw/admin/Technology/GetFile.ashx?fileID=232
IMPORTANT: My past problem was missing the right .SYS file in c:\windows\system32\drivers folder. Get the correct ser2pl64.sys file here:
ftp://ftp.sdt.be/pub/SDT170%20Software/USB/USB%20driver%20Win7/ser2pl64.sys
(this file, only 64000 bytes compaired to the original that was over 150K)
Next I ran the Prolific_DriverInstaller_v1.11.0.exe (from the above zip). Next I copied the 64K ser2pl64.sys file into the Windows8 C:\Windows\System32\Drivers folder (overwrite the original although it may be prudent to first rename the original to ser2pl64.OLD)
Then I plugged in my usb to serial cable, if the cable is not correctly recognized, remove the cable. Now right click the Prolific driver listed under the "Ports (Com&LPT)" listing in Device Manager and click "upgrade the driver"; next highlight "search the local machine" and pick Prolific_3_2_0_0 from the list. Re insert the USB cable.
After many frustrating attempts, this finally worked for me.
Mike K5HUM