http://en.community.dell.com/support-forums/software-os/f/3524/t/19254952.aspx

Ok.  Here’s how to reinstall your Factory Image manually as long as the Factory Image partition is intact.
First, go here: http://www.microsoft.com/downloads/details.aspx?FamilyID=94bb6e34-d890-4932-81a5-5b50c657de08&DisplayLang=en
Download the Windows Automated Installation Kit.  Burn the WAIK ISO image to a DVD using Roxio, Nero or whatever DVD burning software you have that can burn ISO images
Now, insert the DVD in a computer running Vista or  Windows XP SP2 system with KB926044.  Install the WAIK.
Navigate to the Program Files folder where WAIK was installed.  Find the Tools subdirectory.  Under it you will see four folders: AMD64, IA64, SERVICING, and X86.  Copy these folders to a USB drive.  They total less than 100Mb, so you can use a 128Mb USB flash drive if need be.
If you have installed Vista on the C:\ drive, then you can simply boot up your system.  Locate the file name factory.wim on the Factory Image partition. Make a note of the drive letter and path to that file.  On an Inspiron 1720 it is located in D:\Dell\Image\Factory.wim
On eMachines and Gateway computers the file is found in the Recovery partition under Preload\boot.wim
Now go to the command prompt by typing CMD in the Run box.  If Vista is not installed on the C:\ drive you can boot the system with a “live” CD such as Bart’s PE or UBCD for Windows and shell to the command prompt.  Navigate to the Tools\x86\ folder on your USB flash drive.
Type the following command: imagex /apply d:\dell\image\factory.wim 1 c:\
Your system should be restored to its original “as-shipped” configuration in about 30-45 minutes.
Note that this restores Dell’s Proprietary Master Boot Record so that in the future will be able to access the Factory Image by pressing F8 while booting up.