Update: September 10, 2012
Today the hang occurred while the Washington Post was open to a complicated page when I went back into Word to copy the URL. Once in Word, the computer would freeze if I tried to use Word. On restart, it gave the usual warning an invitation to use Safe Mode, which can be skipped.
Microsoft Control Center tells me that I need a fix for a Dell memory driver that is used by some media applications. I just installed it. We'll see if this fixes it.
I fixed the Word file by hard-copying to another file with a different name.