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For system builders with experience the simple solution when moving from a prebuilt system into a new custom build is to buy an OEM release at a much lower cost. You would want a full version that can run as low as $85 or less at this time for the 32bit version of XP Pro.
http://cgi.ebay.com/Windows-XP-Pro-...ryZ41888QQssPageNameZWDVWQQrdZ1QQcmdZViewItem has the actual lowest price found on the full oem version of XP Pro with SP2.
Generally the recovery disks included with prebuilt systems are tied to the one machine whereas the OEM for system builder releases are fully capable of partitioning and formatting a bare drive for Windows to install onto. The price does vary for the oem releases at different sites with some including the upgrade coupon for Vista. Any older version of Windows is simply upgraded with the newer version of Windows when starting the installer while the older version is running even 95. You are always better off with a full version for custom builds since you can reinstall Windows if necessary at any time.