You should extract the files using PSDK-FULL.bat to bew sure all file are correct and valid.
You can use default.htm instead of using Setup64.exe. This particular PSDKs is one on the crappy IE / ActiveX setup nightmares. To install it on a x64 system your IE must also be x64.
If you are lucky the setup works, if not no way to get it installed. Keep in mind the SDK is more that 10 years old out of support, like VC6.
You can also try Windows Server 2003 R2 Platform SDK. Most stuff should also work, and if I remember correctly this SDK has a normal installer not the IE / ActiveX crap.
You can find the ISO here: http://download.cnet.com/Windows-Server-2003-R2-Platform-SDK-ISO-Download/3000-10248_4-10731094.html
Also an hint, this SDK may also work with VC6: http://social.msdn.microsoft.com/Forums/windowsdesktop/en-US/bf785787-8821-4a46-849d-420cf25ed9ad/platform-sdk-february-2003-installation-ie8?forum=windowssdk
From my experience most parts of this SDK are compatible with VC6. If I remember correctly only a few libraries are not VC6 compatible. In this case I used an old lib from an older SDK. I had to do this crappy stuff since a customer wanted a hotfix for an issue years ago...
But keep in mind, also this SDK is long out of support, and did I mentioned that VC6 is also out of support. If you create productive software you should not use it anymore. You should move at least to VC2008.
Note: Posted code pieces may not have a good programming style and may not perfect. It is also possible that they do not work in all situations. Code pieces are only indended to explain something particualar.