Want to have Windows XP, but have Vista right there if you need it? Here's how.

Edit Steps

1
Make a backup of any important data files. It's often possible to do this non-destructively, but if you encounter a problem while partitioning a drive, your data may not be recoverable. You should be making backups anyway, but before major updates such as this is a good time to make sure.
2
Ensure that you have the installation disks for whatever operating systems you want to install. If necessary, also have the serial numbers ready. Also have ready the installers for any software applications you want installed on either operating system.
3
Check the disk allocation. If you currently have one operating system on a single partition spanning the entire disk, you'll need to resize that partition to make room for a separate partition for the other operating system. This may require defragmenting the drive first. In some cases, it's better to have each operating system on a separate physical disk, but usually partitioning works. Check the system requirements for each operating system and make sure each can get a partition at least that large. You may also want to reserve some space for a data partition available to both operating systems. Note that different operating systems require partitions with different file systems. Check the table below for compatibility.
4
Install the operating system to the first partition. During the installation, you should be asked what partition you want to install to. You should also have access to a partition program to modify the partitions of your drive. If you already have the correct operating system on the first partition, skip this step.
5
Install the other operating system on the second partition. The second operating system installer may detect the first on the other partition and configure a boot loader allowing both to boot.
6
If necessary, configure the boot loader. You will probably be using ntloader (Windows) or GRUB (Linux), but check the comparison table for other options. Check the documentation for how to do this. You should be able to set which operating system you want to boot into by default and set a time delay to choose another operating system before the default is chosen.
7
Test the dual-boot configuration. Try booting into each. Note which partitions are readable, and that everything is working properly. Note and troubleshoot any problems.
8
Install any software applications on the operating systems. If you kept the existing operating system on the first partition, any installed applications should still work, but may need to be reinstalled on the new OS.