It could be a corruption in Word's Global Template (normal.dot), it's known to be pretty flaky at times. The best way to cure that is to either rename normal.dot to something like normal.old or delette it altogether. I usually delete it. To that end, close Word and do a search of your hard drive for all files named "normal.dot" (omit the quote marks), sometimes there will be more than one normal.dot file.
When they have all been deleted, restart Word, which will create a new, clean normal.dot file, see if it helps.
Normal.dot is a hidden file, so when searching your hard drive for it, make sure you are viewing and searching in Hidden files and Folders as well. To view hidden files, go to Control Panel > Folder Options > View and tick the "Show Hidden Files and Folders" option, click Apply and OK to exit.