An upgrade form Home to Pro is likely one of the nicest upgrades to do.
Under the hood there is essentially no difference between Home and Pro other
than a few security enhancements and administrative functions that Pro
offers which are not included with Home. As with any upgrade one should
backup important data first. Following this ensure that you have done a
decent disk cleanup and defrag. System Restore Points won't survive the
upgrade so you might as well turn System Restore off through the Control
Pane->System Properties. Critical Updates will not survive the upgrade
either so you will need to reapply those once you've completed the upgrade.
Simply boot to Windows and pop the XP Pro CD into the drive. When the disk
autostarts select Install->Upgrade [recommended]. Then let the installer do
it's thing. During the install of Pro you will be prompted to enter an
Administrator password. Made certain you record this on paper and keep in a
safe place as you may need it later. Ensure that you turn on the Windows XP
Firewall immediately after installing and then get the Critical Updates from
the Windows Update Center.
Info here from Gary Woodruff's page on Upgrading if you need it
http://www.aumha.org/a/xpupgrad.htm