1.Exit Word 2007.
2.Start Registry Editor.
•In Windows Vista, click Start, type regedit in the Start Search box, and then press ENTER. If you are prompted for an administrator password or for a confirmation, type the password, or click Continue.
•In Windows XP, click Start, click Run, type **regedit** in the Open box, and then click OK.
3.Locate and then click to select the following registry subkey: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Word\Options
4.After you select the subkey that is specified in step 3, point to New on the Edit menu, and then click DWORD Value.
5.Type NoReReg, and then press ENTER.
6.Right-click NoReReg, and then click Modify.
7.In the Value data box, type 1, and then click OK.
8.On the File menu, click Exit to close Registry Editor.

Once you have followed the steps above, when you open Word 2007 it will stop trying to set itself as the default version. Simply set 2003 to be the default again and the setting will stick for good.