Top guidelines violations
I just came across the Top Guidelines Violations document on MSDN. This is a great read to get pointers of what not to do with your UI designs and behaviours. It also includes some helpful suggestions as well.
I haven’t read it all yet, but the one that jumps out for me is the restarting progress bar. Hopefully Microsoft will learn from themselves because this is the thing in Windows Installer that annoys me the most. The progress bar restarts so many times and you never know what the progress actually is.