Soooo...
Using process monitor from sysinternals I've watched registry and files access, from starting of setup until the error was given. There's no ACCESS DENIED, only some NOT FOUND for files and the relevant ones are near the end of the log, generated by msiexec.exe trying to open some temporary files from c:\windows\installer.
Those might be some libs the setup unpacks there for calling. Obviously they not there or have other names.
The log generated from setup states that a function from a lib (some .tmp) could not be opened.
These tests were made with Autocad2012 and AutoPLANT SS4, Autocad2010 and AutoPLANT SS3. The results are nearly identical.
I made a virtual machine (VirtualBox) with each setup, starting with vanilla xp sp3.
I'm facing a dead end. Can someone resurrect it?