to be able to convert wim to iso, you just have to do the following:
1. Download first the Windows Automated Installation Kit from microsoft.com/downloads.
2. You also have to make sure that the wim file is located on the root c:/directory.
3. Open a Windows Preinstallation Environment and copy the wim file into it. Type "CopyPE <architecture> C:\Winpe" to create the environment, then "Copy /y c:\discover.wim c:\Winpe\ISO\Sources" to copy the file into the environment.
4. The next step is to navigate back into PETools by typing the command: "Cd C:\Program Files\Windows AIK\Tools\PETools"
5. The final step is to type "Oscdimg -n -bc:\winpe\ISO\boot\etfsboot.com c:\winpe\ISO c:\<imagename>.iso". This will create an ISO image from the contents of the Preinstallation Environment.

I hope each step is clear.