According to at least one person (comrademike's comment), this is a bad idea. It's better to get installation media.
The way I did it was, I filled out the form to get the Windows 7 installation CD (and another CD with drivers) from Dell:
For me (with a 1.5-year-old computer in USA), Dell fulfilled this request for free, and they used overnight shipping (fedex) so I had the CDs the next day. Very convenient!
There are other ways too. The comment by comrademike says it's supposed to be possible to download Windows 7 free from microsoft.com (but you still need a product key; the Windows 7 product key is written on a sticker on the underside of Dell laptops). But he also said that the link may be hard to find on the website. So you may need to contact microsoft to ask for help.
Once I had the Windows installation CD, I just followed the instructions at the other question, How do I reinstall Windows 7 while keeping my dual boot configuration?