XP SP1 locks Win98 users out of MYOB

We are running a W2K Server with a variety of Win98 and XP SP1 workstations. After an upgrade from MYOB to Premier7 if a XP ws logs into the data then the Win98 users cannot. If a Win 98 logs into the data then only Win98 users can follow.The message is “this file is locked”.
We transferred the data to a local Win98 PC and 7 out of 8 users of both types can open the data.One XP user can still not load the data.
Of course MYOB cannot help. There was no problem prior to the MYOB upgrade.
Is there some difference between the way the OS’s lock files? I have looked at a lot of articles but none seem to describe this problem.