Can I repair admin account on SBS 2003 server?

Hi Experts,

I have a problem on a SBS 2003 server.  

It looks like the Administrator account has somehow become corrupt so I can't gain access to the server with admin rights any longer.

I do have one other account which has Power User privileges which enables me to see the properties of the Administrator account but not change the password or anything else.   The only thing I can find amiss is that their is not a logon name (i.e. administrator@domain.local) for the administrator account.

Is it possible to repair this problem and regain control with only Power User access or am I looking at having to reinstall SBS 2003?    

We have only just noticed this problem as the last time we needed the administrator account was well over a year ago - the server pretty much just sits in the corner and acts as an exchange server for remote users.   The problem we have had today is that the security cert has expired so some users who download email into Outlook on their remote PCs can no longer do so.

If it was possible to just tweak the security cert in this instance it would probably be okay.

thanks
Jon