If you partition your hard drive (I recommend an extra two partitions) one for installed software, and one for data, then you may find that if you install a particular application on the application drive, it will record most of the important status details on your operating system drive, certainly in the registry, and often in odd places under individual users.
Install applications into the same directory on the application drive from both OS's may work (a 'repair' installation would be enough). But quite honestly, all you save is the disk space of the application, there is no effective way of avoiding install on both OS that I know of, short of copying the installed details into each registry.
There is no guarantee that installs in different operating systems actually use the same registry parameters, or even install the same files under XP and Vista.
I would say .... impracticable - forget it.
More than that, in the case of Vista in particular, installing any microsoft stuff twice is almost certainly violating the EULA and Vista, the word is, can cut up very rough indeed in cases like this. It can for instance de-validate your software, even (reputedly) your OS
