You Must Install WAIK (Windows Automated Installation Kit) for Windows 7, After this There are a few Ways to do the Same thing, One of them Being this:
Copy oscdimg.exe to root of C:, then Run This Command
C:\oscdimg.exe –bC:\Win7Setup\boot\etfsboot.com –u2 –h -m -lWin7Cust C:\Win7Setup\ C:\Win7Custom.iso
This is the Command Line, for Setup Files Located at C:\Win7Setup\ and The iso Being Created as C:\Win7Custom.iso With Volume name Win7Cust, You Can Change this Accordingly
Also, You can Run This From Deployment Command Prompt, With the Following Code:
oscdimg.exe –bC:\Win7Setup\boot\etfsboot.com –u2 –h -m -lWin7Cust C:\Win7Setup\ C:\Win7Custom.iso