Error on Installing Multibit for Windows 7?
Googling for that error there is a similar one at:
http://trac.jajuk.info/ticket/1549

There they suggest it could be something to do with the installed Java Runtime Environment. This is what the installer uses to run. 

You could try making sure the Java installation on your machine is 100% by installing the latest version from:
 http://java.com/en/download/index.jsp 

Then try the installer again.