This will work provided Vista is installed on your C: drive.
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
Now go to the command prompt by typing CMD in the Run box. Type the following command: imagex /apply d:\dell\image\factory.wim 1 c:\