The golden rule of multi-booting is that the oldest OS should be installed first, and then you progress chronologically. Doing that makes things many times easier overall. It can be done out of order, but it can be a real PITA, especially since XP and Vista/Win7 use different bootloaders.
Fortunately, there's a quick and easy solution to this problem, and it's only one word: Virtualization.
Download a virtual machine program such as VirtualPC Virtual Box, or VMWare and set up as many "guest" operating systems as you want. No multi-booting hassles, no having to restart the computer because you want to use some other OS. Just pick which one you want to be the host OS -- the one you'll spend the most time in, or probably XP if you are a gamer -- and install the others in a virtual machine.
Assuming you have a fairly recent CPU, you should have hardware virtualization support, and you're not emulating CPU architectures, so you should be able to get 80-85% native speed out of guest operating systems. Which is plenty for pretty much all non-gaming tasks.
I had a new computer. I was planning to install Vista Ultimate at the 3rd partition by making my HDD to around 4 partitions. The 2nd one is Windows 7. I just need some help to just boot Vista first and I need some help about booting Vista, but XP first. Which means the booting information should be at XP, right? Because XP is installed first. And can someone tell about booting (dual-boot). The information should be at XP or Vista? Can someone help me?