Installer:
* when starting the install, first wait an initial second to definitely give time for the installed NVDA (that may still be exiting on the UAC screen) to fully exit.
* When deleting each of the NvDA exes, try up to 6 times for each, with a wait interval of 0.5 seconds. this is similar to what the code used to do. It is possible for the installed NVDA on the secure screen to take some time to exit, so we must give as much chance as possible.
