IMAPI is an application included with XP that allows you to drag and drop files onto a CD/DVD writer from Explorer and it will queue up the operation until you're ready to write the CD. Unfortunately, it can interfere with other writing programs like Nero or RecordNow, so it's best to disable it. You can tell if its running by looking in the Task Manager for a process called imapi.exe.
Turning of IMAPI.exe can solve 'Drive in use by another program' problem with Nero. To turn off IMAPI, start up your Control Panel. If you are in "Category View", change to "Classic View" (you'll see the button in the upper left of the screen). Click on "Administrative Tools", then "Services". Scroll down to "IMAPI CD_Burning COM Service", double click on it, check to see if you are on the General Tab, and then Scroll the "Startup Type" to "Disabled", click on "Apply" and you're all done.