If your Dell computer had Vista installed then you can use your copy of Vista CD and you can use the product installation code that is printed on the Windows label that is attached to your Dell.
When you insert the Windows CD into your Dell and boot up your computer then select the CD as the boot device (ie 4=IDE CD).
After Windows is installed and activated then you need to install the Dell computer's device drivers.
You can download the device drivers from Dell WEB site, the support download page. Make sure you select your model desktop or laptop, better still, type in the Service Tag number, it is printed on the label on the bottom of the laptop and on the back or sides of a desktop. (If the Service Tag label is missing then go into the BIOS setting and on one of the BIOS pages, the Service Tag number will be displayed). This will open the page where the specific drivers for your model computer are located. Also make sure you select the Windows of the drivers for your Windows operating system. Click on this link to download the drivers: - http://support.dell.com/support/downloads/index.aspx?c=us&l=en&s=gen