error message when trying to download paid version of Norton Antivirus

Can someone help me - my mother in law has a 6 yr old HP computer with vista 32 bit home version but we bought the Norton Antivirus tried to download it but it gives us an error 'not compatible with your windows version......then lists versions of windows which has vista 32 bit home verson BUT with SERVICE PACK 1 (or 2 etc).

We do NOT have service pack 1 - do we need to go to the windows site to download this windows vista service pack 1 version or update? If so will that solve this problem?  Or do we need explorer 8 (we have old version 7 I think on it now). 

Any help would be appreciated - we paid for Norton but can't seem to be able to download it to use it...thanks for your help.