Since you have no recovery discs or operating system reinstallation discs with your computer, you will need to contact Dell and purchase recovery media. That costs around $15 usually.
I have an Inspiron 1721. The OS needs be reinstalled in this laptop. The product cd key of Windows Vista home premium OEM is at the back of laptop, I just need CD or download link.  Please help  Thanks 
Need installation CD for Inspiron 1721