The least painful is right. For me, that would be to use a utility such as Partition Magic to resize the partitions, adjusting the sizes as you wish. That would adjust the amount of space available on the partition used by each installation, which is what I believe you desire. You could also modify boot.ini in order to change the boot order, making Windows 2003 your primary boot. For information on that, click here.

My second choice would be to backup all files, wipe all partitions, then start over any way you wish. That's certainly a long and tedious way of going about it, but still a valid method.

Hope this helps,
John