Well, I'm thinking of getting Vista but I still want XP. I do have 2 HDDs but my second one only has 2GB left. Is there a way without partitioning to install two OSs without partitioning?

Well, I think you can actually have two installed to the same drive, so long as they use different base directories. I would not recommend it though, partitioning has much less chance of going wrong.

Personally I wouldn't bother with vista at all for at least another year though, even if you can get it for free.

Yes, with a but. No, with a maybe.

Technically, dual boot is no different whether you have OSes in different partitions or not. You just give a different folder name to the other OS (if you have that option) and they will set themselves up and when you boot you'll get to pick. Or at least this is from my experience.

However, it's a very dangerous procedure. OSes aren't designed to avoid each other. As an example with Windows XP, since it's my experience, you can set each installation to use a different folder, say, C:\WINDOWS and C:\WINXP.

However, Windows still has plenty of other folders: C:\Program Files, C:\Documents and Settings (possibly renamed in Vista) and will not check if it will cause conflict. If they both install files on the same spots, you'll end up having two OSes sharing files and breaking each other because the files are not what they need. And you yourself might start mixing up XP and Vista apps and just cause a bloody mess.

If you do want to have both XP and Vista, it's always best to avoid trouble and set them up in different partitions. Partitions don't have to specifically be different HDDs, if you can afford to split your first HDD into two partitions, do it. (there's some margin for a virtual partition to be bootable, any partition manager should fill you in on that)
You can always use the Vista partition strictly for Vista and not install anything else on it so you could size it by the requirements.

If you do want to have both XP and Vista in one partition, be aware of the risks. I've never used Vista so it might be impossible altogether, or just really dangerous and eventually result in breaking both OSes.

I don't have any experience dual booting windows, but I thought the windows installs talked to each other and worked something out. You know, so that you can get a menu.

For the reasons SupSuper has explained, a partition is almost necessary. It can be done, of course, as I did during beta testing Vista. But it was a mistake, and I ended up reinstalling XP to fix the problems.