Select a certificate when trying to access network share

Hi,

I am trying to access shared files (map network drives) on our SBS 2003,
however Vista keeps asking for a certificate. It asks "Select the certificate
you want to use" and then wants me to chose from an empty list.
The same occurs when I try to put "\\server" in the address bar of Computer.

It was working fine. I was away with the laptop abroad and have re-connected
it to our network via Wireless. However, I have done this many times before
without any issues.

Any suggestions are appreciated.
With Tks
Nish.