If you got a Dell machine with Vista preinstalled - Vista would already be activated.
You need a special key for Win 7 - not the one from the side of your laptop - that is for Vista only.
Any one of these will do :
Windows 7 Beta Product Keys
Press the Windows and Pause/Break keys together - the System window opens - click Change product key ( in blue at the bottom of the window)
Enter the Windows 7 product key - click Activate