Two things to try...

1. Try starting Word in safe mode. Hold down the CTRL key as Word is
starting, and say YES to starting in safe mode. Does live preview work now?

If so, then the problem is some kind of setting, registry corruption, or
normal.dot corruption. The steps in the following were designed to diagnose
and repair such problems in Word 2003 and earlier, but I've seen them work
for Word 2007, too:

http://word.mvps.org/FAQs/AppErrors/ProbsOpeningWord.htm

If this does work for some reason, I don't know of comparable repairs for
Excel and PowerPoint. Since the problem affects all three, however, I'm not
optimistic that this will fix it in Word.

2. If safe mode doesn't  fix it, then try creating another User (Start >
Control Panel > User Accounts), and see if live preview works for the new
user. If it does, then it's some kind of user setting that's making live
preview not work. Identifying which one, however, can be a nightmare. I've
often just given up searching and set up the new profile as my working
profile, then deleting the old user once I'm sure I've migrated everything.
If you go this route, just keep rechecking live preview to make sure that
some tweak hasn't killed it.

There was a problem in B2TR in which live preview didn't work in several
Excel galleries when Aero Glass is enabled. You might try switching to Vista
Basic to see if this fixes the problem. Let's hope this isn't the problem,
since this was supposed to be fixed in RTM (I never encountered the problem
in B2TR, but it might affect only some display adapters).

Is Terminal Services running?  If I recall correctly Live Preview is one of the items not supported when running Office 2007 under
Terminal Services.

===========
Hi.

Running Office 2007 RTM on Vista Ultimate RTM.  I have WDDM drivers, a
GeForce 7800 GTX, all visual options enabled (so I have Aero Glass,
desktop composition, etc.). I'm using the latest NVIDIA drivers from
Windows Update.

In Word/Excel/Powerpoint 2007, the Live Preview option is greyed out
and unticked.  I can't enable it, and it's not enabled by itself.  The
tooltip says that it's been automatically disabled based on "Terminal
Server settings" (but I'm at the local console, not accessing through
remote desktop), "connection speed" (ditto), or "animation settings on
this machine" (all Windows animations are enabled, and I'm not aware of
any further animation settings).

Live Preview is one of the big things of Office 2007; why is it not
letting me use it?

One other data point; if I *do* connect to my machine using remote
desktop, Live Preview *is* enabled.  That is to say, it *works* through
terminal server, it's only when accessed locally (at the physical
machine) that it is disabled. >>