Cannot Logon after Reinstalling XP Home and Activating

I just did a clean install of XP home on my parents computer and activated.
After I rebooted and went to log in, it says I have to activate windows. I
hit yes then a window comes up that says windows is already activated and
then proceeds to log me off onto the signin screen. I try to log in again and
it puts me in this infinite loop. How do I fix this. Do I have to delete
everything reformat the harddrive and start again?