The last WinSCP version whose installer supported Windows XP was 5.15.9.
The last WinSCP version whose portable executables supported Windows XP was 6.3.8/6.4 beta.

All downloads are on SourceForge:
https://sourceforge.net/projects/winscp/files/WinSCP/

@Anonymouse: I do now know what you mean by "the direct executables".

Coming back with a second reply, if by any chance original posters are still around (or anyone in the future for that matter), apparently only the portable versions from the archives work with these antique versions of Windows, the direct executables are probably all broken from the looks of it (tried around 5 different ones). The requirements page does mention something about this, but solely for the Windows 7 builds rather than all others even older that might be affected, so I feel like the page could've been a little more explicit there. Regardless, I'm glad I got this nice piece of software to work on such an old machine. Keep up the good work!

I'm yet another person who did attempt running the latest recommended installer (6.3.7) and the one you, Martin, had shown in the above screenshot, the 6.1.1 version. I'm on a "Windows XP Version 2002 5.1.2600 Service Pack 3" machine at the moment, and despite my attempts to run either version directly or through all compatibility mode settings available, it would not run. I've attached the picture of the stubborn error that would pop up every single time. If you'd have any idea how to handle this, that would be of great help. Thank you.

@Anonymous27: So are you saying that portable WinSCP 6.6.1 (or better 6.3.7 as the link shows) does not work for you on Windows XP SP2? Or how exactly is my response wrong?
If you still have problems, please post some screenshots documenting, what you have downloaded and how does it fail.

I'm having the same issue as the original poster, getting the "not a valid win32 application" message using the latest portable download on Windows XP. The admin saying "works for me" is incredibly unhelpful in troubleshooting this problem.

@doug526: What is "XP 32 bit legacy"? The current latest version of WinSCP application (6.1.1) works on Windows XP SP2 32-bit.
https://winscp.net/eng/docs/requirements#client

Can anyone specify the latest version that works in XP 32 bit legacy? Current portable download says it "is not a valid win32 application".

Yes it will work on 32 bit OS

As you have found out meanwhile, on old versions of Windows, you need to use the "Portable executables" download:
https://winscp.net/eng/docs/requirements#client

Hi there

Due to several reasons, I still use Windows Server 2003 (no R2) 32 Bits for some special applications.

Question: Most current version WinSCP-5.17.10-Setup.exe does not; which one is the last version that does?

I get a message when I start the installer:

... is not a valid Win32 application

But WinSCP is only available in a 32 bits but not in a 64 bits version. Maybe I'm wrong with that. (???)

Before I used WinSCP-5.13.8-Setup.exe, and that was working.

Thank you very very much for your feedbacks!! :-)

With kind regards from Switzerland,
Jan