Installing ubuntu is very easy even for a noob (easier than installing windows). But if your desktop PC is really slow, it can be that ubuntu will be slow on it, so I'd suggest you install it on your laptop. If you have windows on it, you can install it with wubi. All you have to do is put in your ubuntu cd while in windows, and you can install ubuntu just like any other program inside windows. Then when you reboot, you can pick ubuntu in the boot menu. If you want to erase ubuntu, you can uninstall it inside windows. Really easy.
You can find tons of sites to help new linux users, just turn to your old freind, Google
. If you want to get the hang of ubuntu, I'd suggest Nixie's video tutorials
. She's pleasant to the eye, and shows very useful tricks for those who are migrating from windows to linux. You can check out some of her videos here (and the rest on youtube): http://www.linuxhaxor.net/