Free Copy Of StartUp Disk

I bought a computer years ago that had Windows XP Home preinstalled and of course, it didn't come with a Windows Setup CD. I'm finally at the point where I'd like to do a fresh install. I can't afford to pay my $6 cab fare let alone $100 or more on an OS I already own and everyone I know has Vista. Is there a way to get a free copy of the setup disk, maybe one that's already been used, directly from Microsoft if you can provide proof that you already own a legitimate copy of the OS so you can just use it for the installation part and put in your own license info? If not through Microsoft, what about from the manufacturer, have you known any of them to give you an option like that where they can send you a "dummy disc" or something just so you can reinstall? My PC was made by Systemax.