After a lot of messing about with cheap USB to serial adapters from ebay I realised that you really get what you pay for with these things. If you want to fiddle about for a day trying to get a device made by a non-existent Chinese company with a fake Prolific chipset to work, and don't mind downloading half a dozen drivers from possibly dodgy websites before you find one that works, buy a cheap adapter. If you want something that just works out of the box, buy one with a FTDI chipset like this one from StarTech.
I ran the driver installer from the CD (run the program that's in in the FTDI directory not the Prolific one), plugged in the converter, and there was an RS232 port. As simple as that. No "Windows cannot find the file...", no "You must reset your computer". Brilliant!
The cable is 1.5 m long which is longer than most, very flexible good quality USB 2.0 cable. The business end of the converter has screw posts for standard jackscrews. It's a businesslike black instead of the usual garish transparent plastic.
All in all, I heartily recommend it. I only tried it on Windows XP. I'm pretty sure you would get good support from StarTech or FTDI if you needed it.