Funny file saving and deletion behaviour after Forefront implementation

I experienced very funny behaviour towards file saving and deletion process
after Forefront was installed. When users open up files on the file servers,
make changes and save, some of the times the save didn't save - that is when
user re-open the files, the files were in the state before the changes (same
as the last time the files were opened). If users delete files, some of the
time, the files were returned intact, almost instantly. If users move files
to another directly, some of the times these files were returned back to
their original folders, as if nothing happened.
For files on local hard disk, I found that it also has this behaviour when
the files are large. I tried to delete a 3.6GB file yesterday on my
computer. It showed the deleting progress and the file deleted but almost
instantly, it returned, as if nothing happened. I have to uninstall
Forefront Client Security, restart the computer to be able to delete it
successfully.
Intermittently problem like this is difficult to solve but such behaviour is
disastrous.
I pushed hard to move to Microsoft Forefront but this kind of problem is
really going to kill me.