OldBloke

The confusion is caused by the using words wrongly.
"Primary" does not necessarily mean "the boot partition". "Logical drive" does not mean "every partition which is not the one to get booted".

Primary Partition just means "the normal simple type of partition".

One hard disk can have up to four Primary Partitions. That's because the main partition table has enough room in it for up to four items. Any one of those primary partitions can be set Active in the partition table.

When you switch on the computer, the BIOS finds the boot hard disk and starts the code at the start of that disk's first sector (the MBR). The boot code there reads the partition table to see which partition is currently set as the Active one and boots it, i.e. starts the code at the beginning of its first sector.

If you need more than four partitions, you can make one of the four slots in the partition table be for an Extended Partition. An extended partition can contain many Logical Drives. That's a way of getting around the limit of only having 4 items in a standard partition table.

If you use that method, with up to 3 primaries plus one extended partition containing a few logical drives, the standard MBR code can't boot one of the logical drives directly, because only a primary partition can be set active.

If Ari was able to set his Ultimate partition as active, it must be a Primary partition type, just like his Home partition is. It didn't boot successfully just because it doesn't have the right code at its start - because Vista put its boot code and boot files on the other primary partition. That can be fixed by using the Vista DVD's boot repair, after setting Ultimate's partition active. That always writes boot code onto whichever partition is currently set active, because that's the one which will be booted.

The XP installer and Vista installer do the same as the boot repairer:
It reads the partition table to see which of the primary partitions is currently set active, then writes boot code onto it (some code without a file name in its first sector and also some named boot files).
If you tell the Vista installer to put Windows on another partition, instead of the active one, it makes the boot files on the Active partition point to your Windows partition (which in Vista's case may even be a logical drive). That's how you can install Vista onto a logical drive - by booting it via a primary partition.

It's very easy to resize and rearrange your partitions if they are all the primary type and slightly more complicated to rearrange things if you also have some logical drives inside an extended partition. That's why I said my step-by-step guide above only applies if both his partitions are the primary type.