First Try this:
I would try booting into the CD.. Just insert the CD and Restart your computer..
it then will boot into the CD and then you will be able to install Windows 7..
If that does not work you need to check to see if Dell Supports Windows 7 on your computer..I see you have a Dell Inspiron 1721 I looked at the BIOS update and it does not have a recent update. That is why I ask to check with Dell about this model and see if they support Windows 7 on it..
Hope this Helps..
EDIT: I just checked online with Dell and to what I am seeing (according to Dell) is that they do not support Windows 7 on your current laptop sorry
.. Here is the Link.. Dell Inspiron 1721
I would still contact them about this issue and see what they can do..