I can see that many are suffering from Nero 6's "Drive is in use by another application" bug and I may have a solution, though others have said they fixed it by doing other things so this may have only worked for me. Here's what I did.
1. Uninstall any other system wide ASPI layers you have installed, that is if you have ForceASPI or something else.
2.Download the latest version of adaptecs ASPI at http://www.adaptec.com/worldwide/support/driverdetail.html?sess=no&language=English+US&cat=/Product/ASPI-4.70&filekey=aspi_471a2.exe
now if you read the instructions for how to iunstall it on XP it says to use the install.bat file I did this but when I ran the ASPI check utility that came with it, it said that I had files missing so I ran the main install, not meant for XP, also and it installed the other files.
3.Go into Nero and rename the file WNASPI32.dll to WNASPI32.old this is NERO's ASPI manager once you rename it nero seems to use the adaptec ASPI.
This did it for me and Nero 6 has been a real pleasure to use since then, I don't promis anything though. Hope this helps.