As the other posts have already stated, you should install XP first. If already have XP and just want to add Vista, you will have to use a third party partitioning software. Partition Magic does a good job.
If you are starting with a fresh installation of both, you may use the Window XP setup to partition your drive. Simply delete all partitions by hitting 'D' at the setup screen. You will have to confirm you want to do this by hitting 'L'. Then hit 'C' to create a new partition that is smaller than 160GB (however big you want the XP partition to be). Then install XP onto that partition. When you install Vista, create another partition by a similar process and use the remaining unpartitioned space. Install Vista on this new partition.