Obviously this has baffled everyone.....so in desperation I tries MS..and look what I found!
It would seem that the error message is not very constructive. Apparently there is a limit of four primary partitions on a hard disk and, although you can create unallocated space, you can't effectively use it!
Vista Home on Dell Inspiron was pre-loaded. There is the issue of just what the other partitions are doing and whether one can be deleted, but that seems to be a can of worms!
Maybe I bin Vista and just use XP?