When you want to burn some data on disc, Nero tells you that the CD (or drive) is already being used by another application and thus the burn process is aborted.
This frequently happens when you try to burn data on a rewritable media that has been recently formatted.
This may be caused by Windows, InCD but also the imapi.exe process.
1 - Windows and/or InCD
Before replacing the CD-RW (or rewritable DVD), close all open windows. The goal is to only see the desktop.
Insert the media and wait a few seconds.
The window of InCD should open: close it if you do not want to burn with this software.
Windows will prompt you, about what you want to do with the media inserted (burn with Nero, open the CD in Windows Explorer .. etc. ...). Close this window that monopolizes the media unnecessarily.
You can now burn properly.
2 - The imapi.exe process
Sometimes, indeed, that even when Nero is closed, the imapi.exe process remains active and therefore causes an error at the next launch of Nero.
To remedy this (but make sure that Nero is closed), "kill" (= stop) this process.
To stop a process (Windows XP), right-click an empty spot on the taskbar and select Task Manager then Processes tab, click the line containing imapi.exe and finally, click Finish process. You can then close the task manager.