If the installer is not letting you perform the re-install because it believes the software is already installed most likely there are registry keys left over from a partial install. Please use the microsoft install cleanup fixit tool at the below link
http://support.microsoft.com/mats/Program_Install_and_Uninstall
1.) Click the "Run now" Button in the Fix-it box
2.) Accept the License Agreement
3.) Choose the option "Detect Problems and let me select fixes to apply"
4.) Select "Uninstalling"
5.) When the list of installed programs populates, select the one from the list you are unable to Re-install and then click next and yes
6.) When the tool completes There may be remnants left so please delete the directory where the product was installed