Windows setup claims the installation source is not accessible. How do I fix this?

There could be 3 reasons for this error:

1. Partition problem described in KB939727
2. Problem with the media or the bad iso/Win 7 installation (burn using another device or using another software; don't use RW media). Use the genuine Windows 7 Ultimate

3. You can try the following:

   • Boot into Windows 7 installer DVD
   • Select Repair your computer
   • Select the Command prompt in the Choose the recovery tool dialog
   • Type diskpart
   • Press Enter
   • Type list disk
   • Press Enter
   • Identify the number of your target HD
   • Type select disk 0 (assuming that your disk is 0)
   • Press Enter
   • Type clean
   • Press Enter
   • Type exit

Now your MBR is cleaned up and you should be able to install the Windows 7