You don't need to buy a new Windows 7 DVD.
You should be able to access your OS restore image that is stored on a partition of your HDD.
It is accessed by pressing F8 when first starting your computer and used to restore the system to factory ship status, as it was when you first got it.
Be sure to move/copy/backup all data that you don't want to loose.
This will reformat your C drive, everything there will be lost.
You should be able to burn DVDs from that partition for re-installing.
You can also request backup disks from Dell, I've gotten them before will no problems. Request Backup Discs For US Customers Only
How to information: Restoring Your Dell Computer to Original Factory Installation with Dell DataSafe Local Backup 2.0