i'd like high definition video

Hi,

If I understand your question correctly you would like to use DVI connection with your laptop so you can get High Definition videos right? Based from the specification of your laptop it only has a VGA port so you can connect an external monitor to it but other than that there are no other ports for you to connect a DVI.

Here's the specification of your laptop and it's user manual that may help.
http://downloads.dell.com/Manuals/all-products/esuprt_laptop/esuprt_inspiron_laptop/inspiron-1721_Owner%27s%20Manual_en-us.pdf