Updating apps

With the release of Office '08 comes a question I've always wanted to ask about updating "messy" apps. By "messy" I mean apps, like Office or Adobe, that leave files all over the hard disk.

I'm considering updating Office 2004 to 2008, and Photoshop Elements 4 to 6 (when it comes out). Do I just run the installer in each case over the existing versions, and assume that the installation will update those strewn files it needs and eliminate the ones it doesn't? What I'm afraid of is that the new version would install parallel to the old version, leaving the old version's files strewn all over the hard disk, and that I wouldn't know which file belongs to which version. Or is it better to first uninstall the old version, using spotlight or easyfind to find and root out all traces of the app? This would be difficult with Adobe because I also have Adobe reader, and I've no idea whether files belong to Reader or to PS Elements.