Installing more than one distribution
 

I'm sure it's possible to install more than one Linux distribution on a single PC, but would this occur without disturbing the existing Linux and Windows installations? And would the installation setup for what would be a second Linux distribution take care of everything, or would files have to be edited so it knows where each OS/distribution is located on the hard drive?

Should be OK. Only snatch is configuring your bootloader correctly. It will probably include windows again correctly but 'm not sure whether it will automatically include any preexisting linux distros

It's actually very easy. A great thing about it too is that you can share mount points between them. For example you could use the same /boot and the same /home (which is great for keeping your settings no matter which distro you happen to be in and for running the same kernel between them as well) and you can use the same swap partition you set up for the other distribution. (Note: it is very important that you KNOW where your existing data is and where you want to install you new OS.) As for a exact step by step how to, if you need it let me know. Google is your friend.

You've got 100GB of storage... even if 40gig is taken up with windows stuff, games etc, then that leaves 60gig ... @ 5gig for a full install of the big distro's = room for at least 12 OS's!!

How much more fun do you want? :D

Do read up on configuring GRUB or LILO, and plan your partitions beforehand...

Yeah, I plan to have lots of fun!

As for sharing /boot, I thought I'd have to configure my bootloader to pick the right kernel for each installation, even if they were the same (kernel). Since the kernel doesn't reside on the /boot partition, how could I use the same kernel for two (or more) different distros? Wouldn't I have to mount other parts of the system as well?

As far as using the same kernel for 2+ distros, I'm not sure that is possible.

If you install a second distro all on a new partition, you have to skip the part where you install a bootloader. You want your original distro's bootloader to remain untouched. You then boot into your existing distro, mount the partition where you've just installed the new distro to and copy the contents of the newly installed distro's /boot directory into the shared /boot partition, preferably in a subdirectory, and configure your existing bootloader so that it knows there is more than one kernel available. For example, an entry from my /etc/lilo.conf:
#####################################
image=/boot/suse/vmlinuz
label="SuSE-8.2"
append="hdd=ide-scsi hddlun=0 splash=silent showopts"
root=/dev/hda8
initrd=/boot/suse/initrd
optional
#vga=0x31a
vga=792
#####################################

The first line identifies the kernel to the bootloader. The fourth line tells the bootloader on which partition the kernel can be found. The fifth identifies the initrd file.
You can repeat this as often as drive space will allow.

Either that or DO install the new boatloader and append similar lines for the existing OS