Long delay accessing XP Briefcase under Vista

Hello
I have a Briefcase which I created under XP. IT is stored on my portable
HDD.
When I work on this briefcase using an XP system there's no problem.
However if I try to access any document in the briefcase using a Vista
system then it takes an extremely long time to open. Even to browse the
documents in the birfcase or to open a 16k txt document.. it takes a very
long time.
Because of my work I must carry always several GB of data between several
PCs so I have to use this briefcase.
I've tried in lots of PCs and always same result.
I think this has something to do with Vista access permissions that's why
I'm posting here.

I tried other utilities like SyncToy but still found Briefcase much more
suitable for the kind of work I do.

Any ideas?
Regards
e.