I hope this info will help those like myself that have an end of life Prolific USB/Serial programmer that will not work in Windows 8.1.
After much searching and more trial and error then I want to think about, I found the following site.
I UnInstalled previous drivers. Then cleaned the registry with CCleaner.
I then followed the linkinstructions and installed the PL2303_64bit_Installer file and rebooted.
Then checked the Comm Port with the Device Manager and changed it if need.
I am now able to use my "OLD" new end of life Prolific programmer.
Check the driver file in \windows\system32\drivers. It should be ser2pl64.sys 9/24/2008 5:03 PM 90KB.