How To Install old versions of Altium on Windows 10

If you've tried to reinstall an old version of Altium on Windows 10, you're probably gotten a problem with these 2 DLL files:

• DNSAPI.dll
• WINNSI.dll

We found the solution here: https://answers.microsoft.com/en-us/windows/forum/windows_10-performance-winpc/windows-10-dnsapidll-and-winnsidll-problem/af7fc4e8-4ea9-4234-b597-a314eb1255e8?page=2

The thread is funny because the idiot microsoft guy keeps telling the customer to reinstall windows, after a fresh install, and the customer get's angrier and angrier, telling the guy that's not the problem. Fortunately, another user posts the solution -

We had this problem with Altium Designer Summer 2009. We were able to resolve it by moving nsi.dll out of the Altium program files directory.It looks like there might be a DLL naming conflict. It looks like dnsapi.dll chains into nsi.dll and it was chaining into the Altium provided one instead of the Windows one."

I removed the dnsapi.dll and was able to get through the sign in and license activation of Altium, and it worked great. You can also put the dnsapi.dll back after you've activated, but you will get some unimportant warnings on start-up.