Windows EFS problem

Hi all.
I used to share a USB key between my home PC (Win 2003 Server) and my
work PC (Win XP, joined to a domain).

To protect my data, I encrypt them via the EFS that is implemented into
the NTFS filesystem.

To do this I have exported the EFS certificate from my work PC to my
home PC and until some days ago all worked fine.

So I had my home PC able to open the encrypted files that I created at
work on the USB key.

But the company upgraded XP to Seven, and now I have no longer access to
any encrypted file from my home PC.

I have checked and it seems that Seven has generated another EFS
certificate, so I exported it (including the private key) and imported
it to 2003 (my home PC).

But,... still I do not have access to any file.

I do have exported the private key related to this certificate but,...
Win 2003 continues to deny the access to the encrypted files.

Does anyone know what I have to check to enlighten this problem?

BTW: if I import this new certificate to a *new* installation of Windows
(I have done this experiment via a virtual machine) all work flawlessly,
and I can open all the encrypted files.

Really, I believe I'm missing something... but what?

Thanks in advance.

Mauro.