Certain older 32-bit applications (notably Adobe Creative Suite CS2 apps, but there are probably others) object to being installed in “C:\Program Files (x86)\”, which is where 64-bit Windows wants to put legacy 32-bit apps.
The solution is easy but non-obvious, as is true so much of the time.
When these programs install and ask for an installation path, edit it. Change “C:\Program Files (x86)\whatever” to “C:\progra~2\whatever.”
This doesn’t work if you’ve disabled 8.3 filenames for performance, but that tweak breaks so many other things (including major antivirus programs, and even some Microsoft programs) that I stopped doing that tweak years ago. It’s just not worth it. Yes, I see the irony in me, of all people, saying a performance tweak isn’t worth doing.
So if you’re trying to avoid upgrading still-useful software to a current version at great expense, that’s the trick that often gets these programs running on a modern 64-bit system.
I can’t guarantee this works for anything but Adobe CS2 apps (I researched this when I mistakenly thought Adobe had released CS2 for free earlier this week), but it’s likely to work for other programs too, so it’s a useful trick to file away in case you ever need it.