If anyone still can't get this to work, shut down your computer and restart it, except push F8 and open safe mode with networking, since it doesn't allow many processes to be run on safe mode it works every time for me on vista.
For me it was a "system" process that was listening on port 80. So, being system, i couldn't close it or computer would crash (and did when i tried). Easiest fix for me was to boot computer in Safe Mode with Networking. Then when you run Charles you should get no problems at all since only core system files are running. This works for me on Windows Vista.