How VSS work in XP

Question about backing up data in XP, I shared all the document folders from
other XP computers, and make sure their VSS and MS VSS Provider services
started, I then mapped these shares on a XP computer. I ran NTBackup.exe on
this XP Pro SP2 machine daily, however when I checked the backup report next
day, some of the files didn't backup because it was in use by other
processes, e.g. outlook pst file.

I thought the VSS can help solve file locking problem, but it didn't work
for me. did I miss out anything? especially, I know if it is a
client-server environment, you can see "previson version" tab, but it's not
available in my situation. Can anyone help?