^+1 for imav. Just decide a place to install your softwares and then install them in both OSs at the same place. Almost always works. But you'll have problem while uninstalling, for that, first make a copy of the installation folder of the application you want to uninstall, then uninstall. After unistall completes, place the copy at original position, boot the other OS and uninstall from there.

When i had both XP and Vista, I had a partition with a separate Program Files folder and i used to install all my softwares there, those which i wanted to share between the two OSs.