While I can understand people's distance (aversion?) to using the Registry, I cannot see how --successfully-- some details can be handled.
1) For example, unless you use the Registry, the only way to find out what other versions of FPC or Lazarus have been installed is through disk scan --or, by asking the user to point the installer to those installations. Even though this could be simplified somewhat by having a central file that keeps a note of those installations, I am not sure it would be the best way to do it --using the Registry seems a better alternative, if only to keep these minimal information.
2) Even though the current Lazarus installer has served a good purpose by also installing a FPC that it is guaranteed to work with, there should really be two installers; one for Lazarus, the other for FPC.
3) Now that the new installer (when it happens) will make sure more than one version of FPC or Lazarus can coexist in the same system, there should be a utility that helps the user to choose which version of FPC/Lazarus should be the default application for *.pas, *.pp, *.lpr etc.
4) Uninstaller should respect Windows conventions; i.e. it should clear everything it installed (see paragraph 3 too, it it's the default application) and offer to clear also the files/folders it generated (i.e. the *.o files etc.). Plus, users should be able to uninstall these from Control Panel's 'Add/Remove Programs'.