BOAC,
The official MS position is that there is indeed a supported upgrade path from XP home to XP pro.
Microsoft Windows XP Home Edition can be upgraded to (and with):
- Windows XP Professional Retail (Full) Version
- Windows XP Professional Upgrade Version
I would ALWAYS recommend a clean install of XP. However, if you make all the necessary backups (and use your new big hard disk to store a disk image for good luck), you could try an upgrade in situ. If it all works, fine. Otherwise you can just do the clean install.
And as BigEndBob says, download all the drivers before you start.
HTH
SD