Hi,
Please check the following articles:
http://windows.microsoft.com/en-au/windows7/installing-and-reinstalling-windows-7
http://www.youtube.com/watch?v=pf408Rkc5IA
http://pcsupport.about.com/od/operatingsystems/ss/windows-7-clean-install-part-1.htm
http://www.buildeasypc.com/sw/windows_7/install_windows_7.htm
and many more. For boot device not found, you want to check this first: