yes indeed, you can’t change an installed Windows but only your Installation CD (or even a recovery CD in case the manufacturer had not left out important parts)
Here’s the detailed breakdown you asked for.
1. Copy the root directory and the i386 directory of the WindowsXP CD to your harddisk
2. Extract the Bootsector of your WindowsXP CD
3. Change 2 Bytes in i386Setupreg.hiv :
a) Open Regedit
b) Highlight HKEY_LOCAL_MACHINE
c) Menu: File -> Load Structure -> i386Setupreg.hiv
d) Assign an arbitrary name to the imported structure e.g. “Homekey”
e) Goto HKEY_LOCAL_MACHINEHomekeyControlSet001Servicessetupdd
f) edit the binary key “default” and change “01” to “00” and “02” to
“00” g) Highlight “Homekey” and select menu: File -> unload structure
4. Burn your new XP Pro CD
5. Install WindowsXP as usual. Your XP Home Key will work.
Note: You cannot apply SP2 to such a WindowsXP Pro, so step 1.b) might be to integrate SP2 in your Installation CD
Please check the menu-entries as I don’t owe an English copy of XP and have to guess them.