This problem can occur when you have several Office versions installed.
(for example 2007 and Office 2003).When you opened for instance
Office 2007 closed it and now you want to open Office 2003 the installer
will start propably because the installer wants to register the actual
opened version again after you opened another version before.
To work arround this problem navigate to:

- HKEY_CURRENT_USER\Software\Microsoft
\Office\11.0\Word\Options\

Create now here in the right pane a new DWORD entry a name it :

NoRereg

(right click in right pane , new , DWORD [DWord Value] and name it as above)

Doubleclick the new entry and set it's value to 1.

Note:This example is for an installed Office 2007 and an installed Office 2003
version.Remember you need to repeat the mentioned procedure for each
of every installed older Office Version (older than Office 2007).
The in the registry key path mentioned 11.0 folder belongs to Office 2003.
Office 2007 has 12.0 (but normally you don't need to modify this.) For older
(!) Office versions look for 10.0 etc.