I was installing SQL Server 2008 today (x64 bit version) and after the initial checks it does, I got an error about MediaInfo.xml file missing. Looking in the folder where it had unpacked the files, effectively I couldn't find a reference to it. After a very hard session of extensive research, I found that others had run into similar issues. Some of the suggested solutions were to re-install .NET 3.5 SP1. Others said to re-install the complete .NET framework. One guy had re-installed his entire OS and still had issues.
Clearly not being the path I wanted to go down, I decided to try and install the 32 bit version to see if it was a problem with the 64 bit. This one seemed to go ahead with the install. Looking at the decompressed files, I saw that the missing files were there on the 32 bit. I then cancelled the install and ran the 64 bit one again. After decompressing the files, the missing files were also there. I thought maybe I had a bad run and let it continue. However once it did all the checks and I hit Continue, suddenly half of the files were deleted from the folder. So it seems that at some point the installer was deleting the files.
So instead of running the installer bootstrap, I decompressed the files and ran the setup manually. And it worked. So if you get a errors on missing files, decompress the package and run the setup.exe manually and it should work ok.