Please right-click the entry in the Device Manager with the yellow exclamation mark and select 'Uninstall'.
Please navigate to the PicoScope 6 installation directory (C:\Program Files\Pico Technology\PicoScope6 on Windows 7 32-bit) and locate the system folder. Please run the DPInst.exe file and this should install the USB drivers.
With regards to the download for PicoScope 6.8.8, please try downloading the file again - it should be 73.4MB in size. I would recommend installing this version.