[Note: I claim no responsibility for any loss of data over what is suggested.]
This is a quick outline of what you will have to do:
1. If you have one hard drive, shrink the current Windows partition: Start -> Control Panel -> Administrative Tools -> Computer Management. Click on Storage -> Disk Management. Right click the boot partition (usually the largest), i.e. - the C drive - and select "Shrink Volume". It will query how much the partition can be shrunk down. Select how much free space you want from the partition and it will shrink it. Then, create a partition from the free space. Make a note of its size. This is the partition you will chose when you are installing FreeBSD.
Now why would you want to do this instead of using a tool like GParted or the like? Because the ntfs3g driver that GParted uses is sometimes problematic. Of course you can use it, but you may end up corrupting your hard drive in the process (which nobody wants). The simplest and safest way is through using Windows itself.
2. At this point you can follow along pretty much what the Handbook says in terms of what to do and so on. Prepare your installation media and boot from it. Since you want both Windows and FreeBSD to coexist peacefully with each other, your installation will have to be a custom one. In this case, it is very important that you delete only the partition you created earlier in Windows, from the available free space. Then, you can select the auto default slice sizes and proceed as normal as outlined in the Handbook.
And, lastly, regarding the bootloader option, select the FreeBSD version. Why? Previously, you had a Windows boot-loader - which can load Windows partitions. By installing FreeBSD, you have added a new partition system - but it is one that Windows does not recognize by default. What that means is that if you hadn't selected the FreeBSD bootloader, you will have installed FreeBSD, but would have no way of accessing it after installing it. So, for all intents and purposes, it doesn't exist - since you can't use it. However, by selecting the FreeBSD bootloader, it recognizes both FreeBSD and Windows - allowing you the option of choosing between the two (which is what you want).
If ever in doubt, anywhere - consult the Handbook. If that doesn't work, then Google your issue.
However, if you are unwilling to risk the integrity of a working Windows system against the possibility of a non-working system (which can happen), the easiest alternative is to use a virtualized version of FreeBSD - which is always safe, but slower (but not by too much though, if you have a multi-core system).
