How Can I Re Install Software From An External Hard Drive After Re Installing Windows

I have the windows DVD so no problem there but I will need to back up my software on to a hard drive to reinstall as many do not have original DVDs. My files are all backed up on a WD 1TB book drive so I should be able to recover them no problem.

I would welcome any comments on the best way to do this or even if I am silly to do this. My understanding is that I will need to back up all software and then reinstall them (if I still want them) one by one. Will  Windows do this? Please note, I am a user, not qualified as a Geek.