When I download a new version of Lazarus I would like to be able to install in a new folder (the install lets me do that) and create a test version - this is where the install stuffs up - it 'remembers' some stuff from the original install - updates some of it - then neither version works!
In fact different versions of Lazarus should work even if they share the same configuration. Then they affect each other which is not practical, but that is another thing. What error you get?
Are you sure your problems are caused by many Lazarus installations, or are they caused by many FPC installations?
FPC is a compiler, Lazarus is an IDE. They are 2 different things.
You can compile many Lazarus versions using the same FPC, and I recommend you do so. Installing and configuring many FPC instances is much more difficult than doing it for Lazarus instances.
This is analogous with GCC compiler versus QtCreator + Qt library. Have you tried installing and configuring many simultaneous versions of GCC? How did it go?
A real improvement would be to have always separate installation packages for FPC and Lazarus.
Then it would be clear to everybody that they are 2 different things.
On Linux they are separate but unfortunately the Windows package contains both. I don't know why. And people act like dummies. They install the latest massive snapshot package, including FPC, just to test latest Lazarus changes, then they have a conflict with an old .ppu file and they create yet another bug report about "Lazarus does not compile". Uhhh...!
The right way is to install the release version of FPC. Or install FPC trunk, no problem, but do it separately from Lazarus.
Then get Lazarus sources for the versions you want and compile with "make all" or "make bigide". Cannot be much simpler.
The sources you can get with "svn co ..." and later "svn up", or using the nice SVN GUI tools in 2013 spirit. Cannot be much simpler either.
Now we are talking about an advanced programming environment! This is not a phonebook or calculator application.
A person who wants to do an advanced setup with many simultaneous versions of such a programming environment, yet does not want to know anything about source code or revisioning, has a serious attitude problem IMO.