Unknown Error while installing Windows 7 on Windows Vista SP2.

I have Windows Vista SP2 on my PC, am trying to upgrade it with Windows 7 but am unable to finish it successfully. I have inserted the Windows 7 DVD in the ROM, then the process starts and there appearing loading windows files, then after selecting the drive to install Windows 7, i get a message that unknown error occurred and process terminates.

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The message Unknown Error means a wide variety of issues which might be the reason behind the message.1. The Optical drive Read/Write lense should be cleaned.2. The Windows 7 Installer Disk may have problems in media.3. The windows 7 software may be corrupt.4. The HDD in which the Windows 7 is being copied, may have generated Bad Sector.For a solution of the problem that you are experiencing take the following actions.Check the Storage Media, DVD if it has any physical defect, take appropriate action.Check the Windows 7 Installer Software for its integrity.Clean the Optical Drive lense using a lense cleaning diskette.Test the Optical drive if it is having any problem or not.Test the HDD using Chkdsk utility to find out wheather there is any Bad Sector / Bad Track in the HDD present, fix that.Finally Format the HDD and Clean Install Windows 7 for a solution. Don't forget to back up vital data.