http://support.microsoft.com/mats/Program_Install_and_Uninstall


Try clicking Start -> Run -> then type:
C:\WINDOWS\Installer
then click ok

then look for the folder called:
{6956856F-B6B3-4BE0-BA0B-8F495BE32033}
then rename it something like:
{6956856F-B6B3-4BE0-BA0B-8F495BE32033}-old