How do i run windows xp and windows vista on one computer?

Create two Partitions, at least 50gb for vista (it uses 15gb itself)
You have to install XP first, and vista later.

Or two harddrives, but the same applies, XP first, Vista second.

(Unless you have both harddrives active, and choose to boot different drives through bios, but that's overkill)

It's better to install XP on one HD, and Vista on other. When you install XP turn off second HD, and when you install Vista, turn off first HD. What you'll have to do, is to manage from BIOS which HD you want to boot first (one with Vista, or one with XP). Thats better then dual boot, because you don't have same root for vista and xp.

There are several ways to switch operating systems:

1. Physically switch the disks
2. Switch disks in the BIOS
3. Use a software loader/switcher
4. Virtualisation

1. Means you either use two bootable external disks, or a swappable caddy system.
This is pretty foolproof as the systems are completely separate, but you can't share data between systems (unless you have a third data disk). It also requires you to buy some hardware.

2. Means you have to interrupt the boot sequence and fiddle in the bios each time you change OS.
However, it means you don't need any extra hardware or software.
One of your drives (say Vista) will be the preferred boot disk. This will be able to enable both drives when using that OS and hence access the data on the other drive. However, that would mean that you can't see the data on the Vista drive from XP.

3. There are several boot selectors, some of which are free. They give you the choice at startup time from a simple menu. There is one in Windows, but they OSs aren't completely separated (as stated above).
You also need to get the software and set it up, and you also need to interrupt the boot sequence to choose the non-default OS

4. I have not looked into virtualisation much, but I have a friend who sucessfully runs her XP sessions in a window on her linux machine. You could probably run XP in a virtual window under Vista (or vice versa).
The main problems here are a performance hit, setup time and also you would need to find out if your Windows licences cover this sort of thing.

To be honest though, I think you're better off choosing one OS and sticking to it unless you really have no choice, otherwise you have a load of hassle - you have to configure everything twice, you'll need to reboot to check your email if you're in the wrong environment etc etc etc

Stuart

You can get Vmware or Parallels and run one the operating systems as a virtual machine.

Here is a good guide to dual booting the operating systems:

http://apcmag.com/5023/dual_booting_xp_with_vista

i use these two versions of the OS on one hdd

it's very easy. you just have to make two partitions. when you install xp you must have other partition for the vista (at least 50 GB). when you install vista it uses it's own boot manager. the only thing you have to do is to show the boot manager which OS you want to start

Use Vista boot pro..
You can then install vista and xp in any order..very handy app.
I'd suggest different hard drives and have vista as drive c and XP as drive d.
Programs will install by default into the root drive whichever OS you're booting into so no worries there.

This is the way to go. Most mobo now have a "quick boot" (name depends on mobo mfg) menu. press Esc (EpOx) or F11 (MSI) or maybe other key on other systems to get the menu during post. set the OS u want to boot from normally as the first boot device in the BIOS. when u boot to this OS u don't have to do anything. when booting to the second os, use the quick boot feature to select the drive that has that os. works like a charm. no messing with vista boot loader, which is a PITA to get rid of if u decide not to use it.

yes but vista will overwrite your boot.ini file for XP once it see's the xp partition. You have to use either vista boot manager or Vista boot pro.
If you install vista first and XP 2nd then you need Vista boot pro to add Vista back to the boot options.

You do not need Vista boot pro. You just boot in the Vista install DVD again and do a repair....this will automaticly add XP if Vista was installed first.