Hi Tim,

If your application depends on a specific VC++ (e.g. 2005/2010) runtime then it's well possible that one of the latest Windows update causes this. And of course, a new or updated anti-virus/anti-spyware product (or definition file) can cause this. If the three customers are running the same protection system then you have the culprit

And what you can do is to provide the customers with a specific debug version of your program and let your program write "log" information into a log file (or add support for "DebugView") to see where it "hangs" and try to work-around it.

Hope this helps a bit.