This procedure no longer applies to DFUSec/LCT. As of v2.00 of the LCT, you no longer need to manually install .NET or the WinUSB driver package - the LCT installer will do that for you if it already isn't installed. WinUSB only needs to be installed on Windows XP machines - it is already part of Win 7/8. All you need to do is download and install the LCT.
To install the WinUSB drivers for the LPC18xx/43xx boards, first configure the board for USB boot. Plug in the USB connector to the board (add external power to the board if necessary) and a new USB device (LPC) should appear.
Download the drivers at http://www.lpcware.com/content/nxpfile/lpc18xx43xx-winusb-drivers-dfu-boot
Un-compress the drivers and point the driver installation dialog to the INF file in the uncompressed package. After installation is complete, you should have a new device available for the LPC18xx or LPC43x.