The very first thing you have to do is burn the recovery discs (about 4 single layer DVDs). Laptop has an app to to that, search in Dell folder. This is very important since installation of any OS will require to reformat the hard drive and it will erase the hidden recovery partition.
The next thing to do before installing windows 7 is to download from Dell website all the drivers. If windows 7 drivers are missing try to use drivers for windows 8. Sometimes it works.