No multi-user file access over network/New Windows Security?

[Quantum]

I think that windows added a security protocol so that if someone has a file open on the network it can't be opened else where. I have a database file on the network that won't open Access 2003. On every other computer it opens fine. If no one on the network has the file open it comes up with a security warning and has the option to open the file. Has anyone else had problems with this?

[Quantum]

This is happening on XP Home by the way.

[Quantum]

I was working on a new computer installation at the time and was working on updates. I installed IE7 and it made simultaneous file access impossible. I am pretty sure it was part of a new security setting but I did not have the time to mess around with it. I quickly uninstalled IE7 and my troubles ended.