|You have not said what is preventing you from installing SP2 on XP Home.|
There may be an answer to it.
Ignoring licensing issues you can install XP Pro in preference to XP Home. Yes I would do a clean install (reformat and Install) of XP Pro and not install it over XP Home (I am not sure you can do that anyway without some tinkering, particularly if the XP Pro CD type is different i.e. Retail/OEM/VLK with respect to the original XP Home CD).
It is however possible that whatever is preventing you from installing SP2 on XP Home may also prevent you from installing SP2 on XP Pro (assuming your XP Pro CD does not already have SP2 on it).
☺ When everything else fails, read the instructions.