OK, but that's not a
Word function. The first thing to check is whether it words with the Windows logo key
+Left Arrow or Windows logo key
+Right Arrow.
There have been a number of reports of problems with this feature (which is a Windows feature,
not a Word feature), due to application issues. Repairing Word might help (Word Options|Resources|Diagnose), as might re-registering Word with Windows via Star|Run and inputting: winword /regserver