To convert and upgrade Windows XP Home to Windows XP Professional, follow procedure below.
1. Open Registry Editor (regedit).
2. Navigate to HKEY_LOCAL_MACHINE/SYSTEM/ControlSet00X/Control/ProductOptions, where ControlSet00X is the one with the highest number.
3. Delete the ProductSuite registry key.
4. Then, create a new DWORD value and named it as Brand.
5. Set the “Brand” value data as 0.
6. Reboot the system.
7. On boot up after the BIOS screen, press F8 to display Windows XP Startup Menu.
8. Choose Last Known Good Configuration (LNG) and hit Enter.
Found on the net.
And of course a non legal hack to upgrade windows xp home.