If you're in the US, you can order the disc for no charge - it'll be the same version that came with the system.  If you want Windows 7, you will need to buy that at retail.

http://support.dell.com/support/topics/global.aspx/support/dellcare/en/backupcd_form

Reinstalling Windows:

http://support.dell.com/support/topics/global.aspx/support/dsn/document?c=us&l=en&s=gen&docid=347576

what if I do not have any of the discs for re-installation.  Does it cost money to get news ones?