Setting the booktype in Nero is only part of the solution to enable DVD-ROM's to be made...
With your burner (NEC 2500) you need to get hold of the beta 5 firmware that Herrie has created...
Then get hold of DVDinfopro and use that to set the booktype for DVD+ discs.
NOTE you can't set the booktype of DVD - or DVDRW - discs ONLY the + types can be changed.
Then using Nero (I use version 126.96.36.199) put a DVD+ in the drive and you will note that when you select to burn the disc you have a check box that allows you to tick it that selects DVD-ROM for the booktype.
After you have burned it you can check the media in DVDinfopro and it will show that the booktype is DVD-ROM