After getting Delphi 2007 to work again I tried to do the same for Delphi 2005 and 2006.
Both versions require the dotNET framework 1.1 which is officially no longer supported on Windows 8 (and 8.1). According to Microsoft, you should contact your independent software vendor (ISV) to have the application upgraded to run on the .NET Framework 3.5 SP1 or later version. Good luck with that.
This is odd, because I could install and use both Delphi versions on Windows 8. The installations broke only when Windows was updated to 8.1, so Microsoft is BSing us here, at least partly.
Reinstalling the “dotNET Framework 1.1 Redistributable Package” failed with some unhelpful error message.
Praise the Google, I found a solution on this site. The .NET Framework Cleanup Tool resolved that problem. After running it, letting it clean up the mess the update apparently made of the dotNET Framework 1.1 and rebooting, I could reinstall it.
Delphi 2005 started again, complained about something regarding “Star Team” which I just ignored / disabled, and behold, the IDE seemed to work.
The same with Delphi 2006. It too complained about “Star Team” which I again just ignored / disabled. It also seemed to work.
Beware: I haven’t done much more than starting both IDEs and compiling GExperts with it. So there might still be issues.