I've run many dual-boot systems in my time, and I've ultimately decided that it's just a pain in the ass, and more trouble than it's worth - unless you just want to experiment to learn stuff...
Nowadays, unless I plan on using the computer as some kind of server, I put some form of Windows on it (XP, Vista or 7).
Don't get me wrong - I loves me some Linux graphical desktop environment (X), and if all you plan to do is surf the web on the computer, you could go for Ubuntu - if the machine is less than 1.0GHz, I say use Fluxbox instead KDE or XFCE - it will run much faster! And you'll get to do some learning, too!
But if you plan to play newer games at all, or want to be able to run the latest/greatest PC software, save yourself the headache and go with Win XP or newer...
Wine is friggin' awesome, but still, I've only had about a 60% success rate... and games still don't run as smoothly as they do in Windows...
My cousin and others out there are die-hard Linux fans, and will argue that Linux can do anything Windows can do better and faster... and maybe it can... but sometimes I just want to be able to point and click and be able to use my hardware/software w/out having to edit any configuration settings, install additional libraries, etc etc... And Windows allows me to do that... most of the time...
I don't believe that one OS is superior to the other, just different... using Linux on a desktop
is a great way to get familiar w/ the UNIX environment, and I definitely recommend using it if you plan on working in IT later in life - many businesses run UNIX/Linux based servers. But if you just want a standard living room computer to play games on, browse the web, type documents and share photos on, then I would just go with Windows...