The unattend answer file contains an invalid product key

dears,

iam trying lite touch installation, but i got this error message

"The unattend answer file contains an invalid product key. Either remove the invalid key or provide a valid product key in the unattend answer file to proceed with Windows installation."

 i tried all these solution on the following link but in vain...

http://blogcastrepository.com/blogs/mayki/archive/2009/04/05/deploying-windows-7-vl-with-mak-multiple-access-keys-using-mdt-2010.aspx

anybody has a work around

appreciate your assistance

M.Zakaria