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The reason your computer is trying to save as a .nrg file is because you have the "Image Recorder" selected as the default CD burner. To change this, to to the "Recorder" menu in Nero and click "Choose Recorder". Select your HP drive in the "Choose Recorder" window. Before you click OK in the "Choose Recorder" window, check to see if your burner supports overburning. If it doesn't, you obviously will not be able to overburn.
If your drive supports overburning, you'll need to enable this capability. To enable it, go to the "File" menu in Nero and click "Preferences". Select the "Expert Features" tab in the "Preferences" window, and check the "Enable overburn" checkbox. Then click "OK".
Finally, burn the image to the CD. When you burn, make sure you select "Disc-At-Once" as the Write Method. Nero can't overburn if you don't use "Disc-At-Once".
I hope this helps!