Ubuntu isn't the best choice for your system; it needs 512MB RAM or more really.
Xubuntu, as you've found, is far better suited.
You're best sticking to one of the mainline distributions (i.e., Ubuntu and derivatives like Xubuntu, or even Fedora or OpenSuse).
Once you've downloaded the .iso file and burnt it as an image to a CDR, you can try it in any machine. You may have to set the CD to be read before the hard drive though; if this is the case, change it in the BIOS, by pressing F2 or ESC or DEL or something just as you turn the machine on (it'll say), then setting CD to be read before hard drive. That way, when you turn the machine on, it'll read the Xubuntu disc first.
It acts as a live CD, meaning that it doesn't touch the hard drive inside the machine. So once you've played about, you can take the disc out and restart, and it'll boot back into Windows as usual. Or, you can install from the CD, and it will resize your Windows setup to be able to fit Windows and Xubuntu side by side. This is a more dangerous prospect, and it's well worth backing up your personal files to CD before.
But it's important to just try this stuff --- it's not as hard as many think, and every 6 months it comes on leaps and bounds with ease of use and compatability.
The forums ubuntuforums.org are great too, ask questions there.