I'm leaving for Croatia on June 23,07. Arriving Dubrovnik and leaving Zagreb. Should I have a car the entire time? I plan on travelling from Dub. to Hvar..then split, Zadar, the lakes and finishing Zagreb. Any help????? or must see things.

A guide book (and doing some searches) will help you immensely.
You don't need a car at all based on what little I know from your question. How much time do you have? How much time do you want to spend in each place? Are you into culture, nature, etc.? More specific questions get really helpful answers.

I am doing this trip backwards. I will pick up my car in Zagreb and then drop it off when I arrive in Dubronivck I thought it was useful to allow us to do some stuff off the beaten track (and my mother is not a bus person LOL!)

I'll be travelling for 15 days.
I have the fodor's book which is very helpful. From Dubrovnik did anyone go to Montenagro? If so..How long a drive is it? I booked the Hilton in Dubrovnik, hopefully the location is good. I also want to stay on Hvar for 4 nights to do some island trips. Any ideas on that.
Thanks

Many people here have raved about Kotor, Montenegro. I can't wait to see it (next month). I think it's about a 2.5 hour drive. So, I take it you will have a car? Otherwise, you can book an excursion to visit there.

While on Hvar, you can take boats over to the Pakleni islands for beaches. To visit other islands from Hvar is more tricky because the ferry schedules do not always allow for it. Which islands were you thinking of? Otherwise, having a car on Hvar will allow for some great exploring of the islands (lavendar fields, coves, etc.}