So I went to install Photoshop CS3 on my Vista x64 box at home, and got this nice error:
Internal Error 2739
Gee, thanks for the info.
Anyway I searched online for ways to fix this, and they involved running regsvr32.exe on jscript.dll and vbscript.dll. Unfortunately, it didn't work! However, I found someone online who had fixed it: http://davidgardiner.blogspot.com/2007/02/internal-error-2739-on-vista-x64.html
David had the critical insight, one that I'm slapping myself on the forehead for not realizing myself: you need to register the 32-bit versions of vbscript and jscript, not the 64-bit versions. The Photoshop installer is a 32-bit program, and thus needs access to all 32-bit DLL's and COM registrations. The instructions that are around the web will register whatever is in your SYSTEM32 directory, which oddly enough on 64-bit Windows is where all the 64-bit DLL's are. (And another oddity on top of that is that the 32-bit DLL's are in SysWOW64. Go figure.)
1) Run cmd.exe with Administrator privilege (Start -> type "cmd", then right-click on the cmd.exe and select "Run as Administrator, then click Continue on the elevation consent dialog). This will open a command prompt in C:\Windows\System32 (or wherever your system directory is)
2) cd ..\SysWOW64
3) regsvr32 jscript.dll
4) regsvr32 vbscript.dll
Voila. Hope this helps anybody else who runs in to this.