Without a true XP Setup disc, you won't have the luxury of the Recovery Console. Your other option is to use your PC's recovery disc, applicable, of course, only if your PC originally shipped with Windows XP.
Going Virtual
Microsoft Virtual PC has preconfigured virtual machines for many popular operating systems.
Before you begin, you'll need to back up any files you want to keep. Unfortunately, you can't use any of Vista's built-in backup software for this, because XP doesn't include any tools that can read Vista backups—understandable but a shame. Your best bet is to copy your data onto an external hard disk manually (via a USB or eSATA connection) so that you can easily get it back once XP is up and running. Make sure you back up the entire C:\Users folder, which will catch your Documents directory, anything on your desktop, and application data such as stored e-mail.
Recovery procedures vary widely, so consult your PC's manual for details. Typically, the recovery software wipes the hard drive clean and then places a preinstalled copy of Windows onto the drive, complete with all the drivers and demo software that you got with your PC when it was new. When it's done, hook up your backup drive and copy your files back into position.
