Contact Dell tech support by telephone. Give them your service tag, and they will send a new disk set to the address to which it was originally shipped for about $36.00
If you are not the original owner, you need to register as the new owner on the Dell website, then wait 21 business days for it to become established. When you are listed as the registered owner, which you can check online, you can then order the disc set.
If the computer has been reported stolen, they will not ship the discs, and will notify the local authorities for the address you posted. Then you will have a bit of a hassel establishing how you obtained it. If you obtained it in any way for which you have a receipt, then Dell will still register it to you and sell you the discs.