Here's the rub, you can install as many OSes as you like "PROVIDED" they don't interfere with each other. Now, saying that and doing that are two different things. Trail and error is the most instructive lesson you can learn. You already got a difficult setup now with XP sp1 and sp3 both installed. So, if you don't want your hard work wasted, take it one step at a time. The boot.ini file will have to involved during all this, so I suggest you save the current working one and/or NTldr file to return to before if all fails. In a round about way, if you've tried the "live Ubuntu CD version" you get an idea how that OS will work. You can "sandbox" it using a "virtual setup" allowing its install within its parameters and see how things work. Google more for virtual and check it out. I assume you already have some arrangement now to tell which OS to boot, like the startup menu.
I have my internal hard drive set up with 3 partitions. on the 1st partition I have XP sp1. I use that for a backup operating system. On the 2nd partition I have XP sp3 and I use it as my primary operating system. The 3rd partition is the largest and I use it just for storage. I dont really need it because I recently purchased a large external drive. I really want to try the Linux based Ubuntu operating system. Can I install it on the 3rd partition?