Likely you can't as you have attempted to use the recovery DVD and its upset the factory partition.

Refer to my wiki a clean install of Windows 7 below. The top note of this tells you have to go through with restoring from the factory settings; you can see if it works if you want although I prefer to install via DVD. The rest of the wiki tells you the procedure to install via DVD/USB.

What is the exact error message? Were you reinstalling the way I mentioned in the wiki or doing something different like trying to upgrade? In particular this screen:

Select custom (advanced).

Hi, thanks for your help  i choose the  custom  option  and  i had to install all de  drivers and controllers  by hand as well as the  diigital persona and the  web cam, but my  computer is  far from being  like the fabric option , i called at dell and ask if they could help me  and say it is impossible to doit  and they   try to convince me  that this is  the best option i have done. some how  i'm not  quite  sure ,and  i don't know many things  about computers, so if you know a way to  do it  i will be more than happy to do it. Thanks  again.

Sorry I don't quite understand your problem. You have Windows 7 installed & you have all the drivers installed? Do you have security software installed and all your programs also? Did you recover all your files from Windows.old (or back them up) and delete Windows.old?

You followed the guide "A Clean Install of Windows 7/Windows Vista"?

What is the problem?