Here is the situation in short. I save up and buy myself a nice new $1200 PC from an online store. The store is legit and what not, but they forgot to neglect one little detail. The PC comes with no operating system (despite the page having "Built for Vista" on it and they don't say it has no operating system). I don't have the money right now to buy Vista and my windows XP disk is... Somewhere. (And for that price you would expect them to at least stick an operating system on the god damn thing)

So I decide to get Linux, more specifically Ubuntu (since it is the only one I can remember by name). Question is this, once I get Ubuntu set up on the computer can I in the future (when I have the money) buy Vista, partition my hard drive and have both Ubuntu and Vista on at once (dual booting I believe they call it)? I have seen websites showing that this can be done when windows is initially installed, but can it work the other way around?
If so how would I go about this and are there any disadvantages in doing this?

Man, it seems so foreign to me the idea that people actually buy operating systems. XD

But yeah, when dual booting ubuntu and windows, its always best to install windows first. Not sure if you can do it with ubuntu first. So use Ubuntu for now, once you get vista, you probably reformat the drive, and install vista there. From that point on if you want to install ubuntu, you're best bet would be to use wubi (comes with the newest ubuntu) which lets you install ubuntu without partitioning your harddrive like a windows program.

As far as I know there aren't any real disadvantages, except for you have less space for files, seeing as you have two OS.

Its perfectly possible, except that the windows installer (at least up to XP, I'd be surprised if Vista is any different) overwrites your master boot record without prompting.

If you want to build a dual-boot system with windows added second, I'd advise you to create a sensible partition set to start with (first partition on the disk should be NTFS, then an LVM volgroup for your linux stuff). Before installing windows, make sure you have a bootable disk (a GRUB floppy works well for me, though I suspect most liveCDs or linux rescue disks will do the job) so you can restore the MBR afterwards.

I don't know about ubuntu (I'm a redhat addict), but I suspect it'll give you pretty useful tools to do whatever you need.