PC asks for CD after update to complete update

Hi,
When I try to update or add on some packages (with e.g., synaptic), the download takes place as it should up till about 95% - then it asks me to insert the CD. Only problem - I gave the CD away and cannot get it back. When I then do not insert the CD, the whole update/add is cancelled - RESULT :- NOTHING added/updated of that package. I tried another CD but the PC does not recognise it as CD. - it continues to ask me to insert the CD.

Normal security updates are not affected.
But I have a leaner ubuntu experiance coz I cannot download some stuff.

Is there an easy answer WITHOUT having to over re-install the whole Ubuntu program (I am afraid to do so coz i'm afraid that it may not work and then I will have no linux) Where I live I cannot reach anyone else who knows Ubuntu to assist me. There probably are many other people around but they use Suse or other distros and do not know Ubuntu - I tried making contact with a local group (PLUG in our area) - but THAT is really difficult! It is a programmers group and not accessable for a newbie

Rusty