iCal only works with Exchange 2007 or later. Microsoft uses a different connection protocol in Exchange 2003.
You can try Davmail. I do not use it and I am not promoting it so no guarantees. The posts I'm reading seem to indicated it's a bit buggy, but maybe it's sufficient for you.
Ralistically, your company should be seriously considering upgrading since Exchange 2003 is already past normal support and will go end of life in a couple of years. Additionally, Exchange 2010 now supports Safari for Mac when connecting to Outlook Web Access, so basically, Microsoft really started showing support for Mac in Exchange 2010 (and not just giving Mac lip service.)
I've contacted my mail provider and they've confirmed that although my package is shown as Exchange 2003 in the online control panel, it is in fact 2007. They also asked me to create an Autodiscover SRV record in my DNS zone, which has helped resolve the internal and external addresses more fully when configuring iCal, but hasn't gotte me any closer - it still states that the account cannot be refreshed.
Thanks for your help and patience so far guys, any further pointers gratefully received!