What did you use originally to see 7 installed? If your first install was without SP1 the SP1 iso from the site won't help you any since the product key is pre-SP1 and simply won't see any clean install using the download from there activate!
As for the write to flash drive or the download itself it may have only been a partitial or bad download where a second download would be needed. If the write to flash drive was incomplete using another program like the shareware version of UltraISO
would be advised. After numerous writes for both 7 isos as well as for other OSs that has been reliable.
The other program better for burning iso images to blank dvds has been the Passcape ISO Burner
which will only write Windows isos to flash drives. It will burn any iso to disk however and has been found a good freeware for dvds.