office files becoming read-only

Hi All

Hope someone can help?

I'm having a problem with office 2007 files becoming read only suddenly
while they are open. These files are on a mapped drive to a share on SBS
2003. This is happening with word, excel and project files. They open ok
without being read only. They will save without any problems most of the
time. Suddenly they will become read only and require saving to a new name.
It is really annoying as the only way I've found to fix it is to save as a
different name, reboot my client pc or login again, delete the original file
and rename the newly saved file to the original. I am the only one using
these files and have administration access. The files don't have the
read-only attribute to them when this occurs but they can't be deleted or
renamed.

My PC is XP SP2 and I've installed the recent office 2007 sp1.

Any help would be appreciated.
Regards
ilr