Apparently, there's a nasty little bug in Word 2007 that occurs sometimes when an automatic update is applied. If Word is running while the update occurs, and your machine reboots while word is running, something bad happens to the system registry that basically cripples it. Symptoms include: your mouse no longer responds within the document or it crashes altogether when you start/exit the program.
It's happened to me twice within a few weeks so I figured it was something worth blogging about.
You can find the magic sauce to fix it here.
To summarize the KB article, you just need to delete the following registry key and you'll be fine:
HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Word\Data