Backend accdb locked in networked pcs running Vista Basic

scenario:
2 pcs with Windows Vista Basic in a Workgroup
access 2007 runtime installed on both
backend database located in the Public folder on PC1
PC1 Network and sharing options: file sharing ON; share public folder ON;
shares protected by passw OFF

both Pcs have installed a front end application (accdr) that link to the
database (accdb) in the PC1 Public folder.

in PC1 I can run more than one FE with no problem but if I try to open the
PC2 FE, it hangs until the PC1 FE are closed.
Now, if I open the PC2 FE first and then open the PC1 FE, both FE work fine.
(????) why?
Why the computer that owns the database, locks it to other computers, if he
is the first to open it?

This same scenario runs ok with XP professional.

Any idea?
--
Jose Ribas