Keith22, I had a similar problem with Nero 6 at work but I didn't check My Computer to notice any change from DVD to CD. It might have been there but I didn't look. Bear with me on my long story but I tell it so you can see similarities and differences to your problem.
Anyway, the part of your problem that caught my attention was where you couldn't change the Nero selector to DVD. Me too. In my case, neither of the DVD and CD selection tabs even showed up but it defaulted to CD. I was using a Sony DVD+RW disk that I have used many times before for data backup. I don't close or finish the disk so I can add or delete data. That's quicker than doing a full backup. But periodically, I full erase the disk so I can copy to a clean disk. I usually right click on it in My Computer to full erase (not quick). After erasure I tried to create a data disc as usual but Nero saw it as a CD not a DVD and as I said before I couldn't change it. I tried to save data anyway but it returned an error message about not having the right media (a CD).
In retrospect I did burn a data CD RW as the last project I worked on but I don't know if that had anything to do with it. Evidently it was stuck on CDs and it wouldn't recognize my erased DVD and I'm still not exactly sure how it happened but it did. This part is similar to your problem.
I solved my problem by taking another DVD+RW (a second backup disk of the same data but created separately - not a blank) and opened the existing project and Nero correctly showed it as a DVD. I knew the DVD was recognized by the measurement bar along the bottom that shows how many MBs are used and available. Since I finally have Nero recognizing a DVD+RW, I used the toolbar drop down menu and erased the DVD. I've forgotten which menu item has erase. After this, somehow Nero recognized my blank DVDs again. Even the DVD/CD selection tabs returned.
It might have also been fixed if I added or deleted data on the data DVD and saved it but I didn't happen to do it that way. There also might be better ways of fixing the problem but I was going by trial and error I found something that worked for me. I don't know if it will work for you but maybe it will give you some ideas to try. Of course, you could always try uninstalling and reinstalling Nero. Good luck.
