With windows 7, I plugged in the cable and windows when online and downloaded the driver all by it's self, there was no input from me.
The FTDI for the cable from the ftdichip.com are for all windows versions. If you installed the driver from the CD that came with your cable that is most likely your problem.
You could have a bad cable. One of the TuneEcu threads shows how some of the cables may have bad solder joints. It's fairly easy to take apart to inspect and repair.
If you're not familiar with soldering on a PC board get some help.
Assuming your cable is good, I would uninstall the cable and drivers and delete the drivers you downloaded. Then, with an active internet connection plug your cable into the USB port. Windows should find the new hardware and search for drivers. You may have to manually search for new hardware through the "Control Panel"
The link is for a FTDI drivers installation guide for Windows 7.
Good Luck.
http://botnroll.com/onec/downloads/FTDI_Drivers_Installation_Guide_for_Windows7.PDF