Hi Neil,
Welcome to the forum.
I assume you are probably getting a blue popup window with the message "Windows protected your PC...Running this app might put your PC at risk". This is probably generated because the file is not digitally signed. However, it has been virus checked and is safe to use.
To install it, click on the "More Info" link in the blue popup window and then click the button "Run Anyway" Ignore any message about the dangers of running an executable file.
Windows Defender would not stop the program installing. We recommend disabling Windows Defender while runnning BridgePal in the club because it may initiate a real-time scan when it thinks the PC is idle (no keyboard or mouse activity for a period of time). This scan is very CPU and disk intensive, and on some PCs can cause extended delays in responses to requests from the BridgePal phones/tablets.
John