I have just had this problem and it was driving me mad, I was thinking it was down to Windows Live Writer, which I had just installed but after a little investigation turns out it is a common word 2007 issue, anyway here is the fix.
Delete the Word Data registry key:
Most of the frequently used options in Word are stored in the Word Data registry key. A common troubleshooting step is to delete the Word Data registry key. The next time that you start Word, Word rebuilds the Word Data registry key by using the default settings.
To view these options in Word 2002 or Word 2003, click Options on the Tools menu.
To view these options in Word 2007, click the Microsoft Office Button, and then click Word Options.
Note When you delete the Word Data registry key, Word resets several options to their default settings. One such option is the "most recently used file" list on the File menu. Additionally, Word resets many settings that you customize in the Options dialog box.
To delete the Word Data registry key, follow these steps:
1. Exit all Office programs.
2. Click Start, click Run, type regedit, and then click OK.
3. Locate the following registry subkey, as appropriate for the version of Word that you are running:
• Word 2002: HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Word\Data
• Word 2003: HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Word\Data
• Word 2007: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Word\Data
4. Click Data, and then click Export on the File menu.
5. Name the file Wddata.reg, and then save the file to the desktop.
6. Click Delete on the Edit menu, and then click Yes.
7. Exit Registry Editor.
8. Start Word.
If Word starts and works correctly, you have resolved the problem. The problem was a damaged Word Data registry key. You may have to change several settings to restore your favourite options in Word.