For reproducing this,
- Download IE6 web setup here: http://www.oldversion.com.ru/windows/download/internet-explorer-6-0-setup-only (version tested by me, but for me IE6 SP1 offline setup also affected, so you can use any other IE6 build for the test);
- Open Regedit, go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer and set Build, Version and W2kVersion to lower value than in ie6setup.exe properties window (for example, I tested version 6.0.2800.1106, so I lowered version in the registry to 6.0.2600.0000: Build 6260000, Version 6.0.2600.0000 and W2kVersion also 6.0.2600.0000. After this, IE6 setup is able to run correctly and not reporting that newer version of Internet Explorer is already installed. I think that 5.0 or older version number can be also used there);
- Reboot ReactOS for applying changed registry settings;
- Launch ie6setup.exe, accept the EULA and click "Next".
After this, in 0.4.12-dev-631-g88cd8d681c15fb06bd417985fe2c873df688a3e9, installation continues successfully. Since 0.4.12-dev-632-gd93e516747e3220ba182f77824e8b1a8b548edae, it fails to snap ExtractFilesW in advpack.dll, even with replaced advpack from Win2k3.
In debug.log, I see the following before this error appears:
So problem is in ldr, not in advpack.