Try this from my UnBricking post Bricked my Sheevaplug, Unbricked & upgraded at once -- HOWTO
15. Install the FTDI drivers:
Open "Device Manager". Plug USB Serial cable from SheevaPlug to the computer.
Hardware will be found. Open the libusb-win32-bin-126.96.36.199\bin\ directory.
Run INF-Wizard. Choose the device with "interface 0" in the description.
Install driver. Right click on "SheevaPlug JTAGKey FT2332D B" in the Device Manager.
Update driver, Point it to the CDM 2.04.16_SHEEVA directory.
Right click on "USB Serial Port" in Device Manager, Update driver,
Point it to the CDM 2.04.16_SHEEVA directory.
Hope this helps,