Multimounter does work under xp pro sp2, it's just a bit annoying. open the properties of your cd drive and select the Volumes tab. Each burn session will show up as a separate volume with a check mark indicating the volume presently active and accessible. highlight the volume (session) that you want to access and click apply. You may need to refresh [F5] your explorer's window before the change shows up. It really sucks, but apparently you may only access one session at a time, and not all of the files at once.
I assume this is because of the annoying multisession auto refresh in nero that may have tricked you into burning the same filename in each volume (session). This gives low end users the illusion of truly refreshing a file, which is impossible without rewriting, something you aren't doing with a multisession write once disc. Consequently,there may be multiple files of the same name on your disc, but written at different times, in different sessions, and containing different information. If you tried to do this in explorer it would make you name the subsequent files differently.