I'm using win2k pro sp5 can i do a vista upgrade?

Straight from the horses mouth :)
http://www.microsoft.com/windows/windows-vista/get/upgrade-your-pc-options.aspx

How to dual boot Vista and XP (with Vista installed first) -- the step-by-step guide with screenshots

UPDATED | Got a Vista PC and want to install XP so you can dual-boot between them? Here's how to do it, in an easy, step-by-step format. Page 1 - Intro

Scenario: You want to install Vista on your PC alongside your XP installation, on the same drive. You have installed Vista already. (If you have XP installed first, check out our earlier tutorial on how to dual boot Vista and XP with XP installed first.)
Tutorial Summary: We need to shrink the Vista partition on the hard disk and create enough space for an installation of XP. This can be done in three ways - using the GPartEd Live CD, the DISKPART utility on the Vista DVD or the Vista Disk Management GUI - and we'll cover all three. We'll then install XP and install EasyBCD to reinstate the Vista bootloader which will be overwritten during the XP installation, and then use EasyBCD to configure Vista's bootloader to boot the XP partition.
Updated September 2008: This is an updated tutorial, based on our first Windows Vista/XP dual-booting workshop. The main differences are that we are now using Windows Vista SP1 and Windows XP SP3. EasyBCD has also been updated and this makes some of the steps in our first tutorial redundant - the whole process is now easier.
This tutorial has been tested on a VMWare 6 Workstation.

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VISIT THIS FOR THE COMPLETE TUTORIAL :
http://apcmag.com/how_to_dual_boot_vista_and_xp_with_vista_installed_first__the_stepbystep_guide.htm
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