"I installed a DVD burner to replace my old CD burner (which was not removed from the tower). Why is it when I want to burn CDs, both Windows and Nero chooses my old burner for the process instead of the new DVD burner? How do I fix this?"
The why part is slightly different depending on whether you are using Windows XP or Windows Vista when you burn DVDs and CDs. Vista actually has a drive properties setting that allows you to choose a default burner. Assuming the programs you use respect this setting, the burner you choose should always be the default. Windows XP defaults to using the burner with the drive letter closest to the beginning of the alphabet as do many programs. Depending on which problem you have, the solution is slightly different.
To change the default DVD burner by changing its drive letter, click Start and right click My Computer, choosing Manage from the list of options. In the Storage menu, choose Disk Management, then locate the drive you want to show up second in the list of available drives. Right click this demoted drive and choose Change Drive Letter and Paths. Click Change, then choose a drive letter that's higher than your new drive. After you click OK on all open screens, you might need to reboot before the changes will be reflected in all your programs. This should force all applications on your system to view the new drive as default.
Setting the default DVD burner in Windows Vista is a little easier, although you may need to use the drive letter trick in some cases. To change the default, Click Start > Computer, then right click any of your burners and choose Properties. From the Recording tab, choose the drive you want to be your default recording drive from the top dropdown list. Click OK and in most cases you're all set with a new default.