I'm afraid there is no way to downgrade Windows back to 7. You must backup all your files and then do a clean install (format) booting from Windows 7 DVD-ROM or USB. The message that some files are missing might indicate corrupt download. Try downloading again or use original DVD-ROMs. Also make sure you don't try installing 32-bit Windows if you have more than 4GB RAM, it will fail. You must install 64-bit version instead. Format (wiping the disk) is the only way to go. Any other way will get stuck somewhere in the process as it will try to preserve some settings that are most likely incompatible. You can reinstall all your applications in Windows 7 and restore your data from your backup.
But why you have to downgrade in the first place? Windows 8 is supposed to be compatible with Windows 7 and any software that runs on 7 also runs in 8. Some incompatibilities are most likely due to access priviledges which can be resolved by either disabling User Account Control or right clicking on the program or shorcut you try to run and set it to always run as administrator. In more difficult cases you can also set compatibility mode to Vista or 7, even XP. For some programs or games there are also patches released to make them compatible with Windows 8, see at the application's site. Also all Vista and 7 drivers of the same kind (eg 32-bit) should also work in Windows 8. Again difficult cases are treated in compatibility mode or by manually installing the driver from Device Manager. Don't forget to install DirectX 9 to play games. Although Vista have DirectX 10, 7 have 11 and 8 have 11.1, to play most games you must also download and install DirectX 9 which adds the necessary files and updates files of newer DirectX versions. Needless to say that Microsoft drivers are very basic and to do your job properly you must always upgrade to OEM drivers (eg nVidia Forceware for graphics, Realtek for HD audio etc). You can even add back the missing start button, start menu, desktop gadgets and Microsoft games in Windows 8 (Google it to see how) to make them much more like good old Windows 7. If you use the utility Classic shell it adds the start button, start menu, optional cut/copy/paste icons in Explorer and can also bypass the Metro interface and load the desktop automatically at startup!
I hope that helped!