It's probably because you have convertor that doesn't use genuine prolific chips. The OEM drivers won't work with them. But if you go to the download page of my site, I have some older drivers which don't know the difference and should work.
The software at the bottom of the download page solved my same problem for my Win7 machine, but now I need the one for XP but the links on the FreeScan site are dead.
I was able to google the driver for Win7, but I can't find the XP one anywhere else but your site.
"Windows XP Prolific DriversThese are handy drivers to have if you are using XP or 2000 and the Prolific drivers are producing a “Code 10” error. This frequently happens with cheap $2 USB-Serial adapters. The latest Prolific drivers seem to not get along with them, however, these older “GPS” drivers will work just fine. "
BTW, I've got a cheap cable I bought from MonoPrice and the newer drivers don't work.
Thanks,
Jon