There shouldn't be anything special about installing a dual boot system on a Mini, except for the lack of an optical drive.
I'd do it like this:
Install Windows first by some means, leaving 4GB or more free. Or if Windows is preinstalled, backup any data, then shrink the Windows partition to free up 4GB or more. I don't think XP can shrink partitions, but Vista/7 or the Ubuntu live installer can.
Then boot up the Ubuntu installer. It should setup the Linux bootloader with the choice of Windows or Ubuntu on boot. One tip: I would avoid creating a swap partition on a small SSD