I was installing oracle 11gR2 on windows vista and got the file not found errors.
You need to run the deinstall batch job first(located in <oracle_home>\deinstall folder), since the install failed the first time and comes back with error.
Then extract files from both zip files into the same folder.(There is nothing to overwrite since the contents of both zip files are different.When it asks if you want to merge the folders, give yes and proceed).
In previous versions of oracle, it used to ask for the second disk location in the middle of installation so there was no such problems.
By default, when the 2 zip files are extracted, it will show 2 different folders to be extracted, by default. So nobody really notices this. There is nothing about this in the documentation too.
The solution that we need to manually merge, worked for me but as others say, I am really annoyed that oracle didnt find this problem and fixed it.Such a shame Oracle!!Now I dont think you as a world leading software company.