Uh-this is A LOT easier than you think. However, I have only dual-booted with Ubuntu and XP, but Freespire will be similar.
First, the distro (in this case Freespire) will install everything you need to operate (hence operating system) including the kernel.
Download and burn copy of GParted
. Don't worry about losing a CD, you may need GParted in the future. For tips on burning it, look at the section in my sig about basic Linux questions.
Once that is done, install Freespire, have it take over your whole drive. Then, don't worry about setting things up, just reboot with the GParted CD in your CD tray. Once GParted boots up, it will open a graphical partitioner. Resize the "ext3" partition (the biggest partition) so that half your drive in 'unallocated'. Then click "apply". This will make your partition, so when it is finished, double click on the poweroff button and shutdown.
Then, reboot with Windows in your CD drive and have it install on the free space in your drive. If you don't have a Win CD, then you will have to set everything up without uninstalling Windows, which I don't know how to do. (other people on here do).
If this does not help you, reply to this, and also do a search in this forum for dual-booting. Also, put up your specs for your computer.