I am desperate to disable "Welcome back pick up where you left off". This feature means I cannot search Word files without pressing the search shortcut three times before it responds. I am a researcher and do scores of searches through my files of notes
every day. My RSI has recurred as a result.
It does appear that you have to wait for the Welcome Back prompt to "collapse" to
 before Word responds properly. I hadn't noticed.
For now, all you can do is lock down the registry entry as discussed earlier in this thread. (You need a version of Windows that supports it.)
You should post to http://word.uservoice.com and suggest that an option is introduced that lets us disable Welcome Back in the user interface.