I would suggest downloading the boot-only mini-CD image from here:
and doing a minimal install after booting from that (it will allow you to install from an FTP site, such as that one). That will take up a whole primary partition ('slice').
During installation, partitioning is done in two stages: first, a primary partition is created or selected to be a FreeBSD slice, then on the next screen you can set up the partitions within that. There's a default option for creating those partitions, which should work fine.
I'm not sure if the root directory (which includes /boot ) is necessarily in partition 'a' of the selected slice.
If you are setting up to run on a system with Windows XP on it, the file /boot/boot0 can be put in the Windows C:\ directory, and the NT/XP BOOT.INI file can be edited to load that in order to boot into FreeBSD (apparently that works if the FreeBSD partition is on the same drive - I've done that, myself).
BTW you can also mount Linux partitions from FreeBSD; one problem, at least in earlier versions, is that the inodes had to be 128 bytes in size - that may have been fixed in FreeBSD 7.2, but I haven't tried that yet (that's part of the reason I set up this test system, but I got sidetracked on the GRUB 2 issue, when I saw the Karmic alphas had come out and tried them!)